Lithium-manganese composite oxide, secondary battery, and electric device

ABSTRACT

The amount of lithium ions that can be received and released in and from a positive electrode active material is increased, and high capacity and high energy density of a secondary battery are achieved. Provided is a lithium-manganese composite oxide represented by Li x Mn y M z O w , where M is a metal element other than Li and Mn, or Si or P, and y, z, and w satisfy 0≦x/(y+z)&lt;2, y&gt;0, z&gt;0, 0.26≦(y+z)/w&lt;0.5, and 0.2&lt;z/y&lt;1.2. The lithium manganese composite oxide has high structural stability and high capacity.

TECHNICAL FIELD

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. In particular, oneembodiment of the present invention relates to a semiconductor device, adisplay device, a light-emitting device, a power storage device, astorage device, a driving method thereof, or a manufacturing methodthereof. In particular, one embodiment of the present invention relatesto a structure of a secondary battery and a method for manufacturing thesecondary battery. In particular, one embodiment of the presentinvention relates to a positive electrode active material of alithium-ion secondary battery.

BACKGROUND ART

Examples of the secondary battery include a nickel-metal hydridebattery, a lead-acid battery, and a lithium-ion secondary battery.

Such secondary batteries are used as power sources in portableinformation terminals typified by mobile phones. In particular,lithium-ion secondary batteries have been actively developed becausecapacity thereof can be increased and size thereof can be reduced.

In a lithium-ion secondary battery, as a positive electrode activematerial, a phosphate compound having an olivine structure andcontaining lithium and iron, manganese, cobalt, or nickel, such aslithium iron phosphate (LiFePO₄), lithium manganese phosphate (LiMnPO₄),lithium cobalt phosphate (LiCoPO₄), and lithium nickel phosphate(LiNiPO₄), which are disclosed in Patent Document 1, has been known.

Layered rock-salt compounds such as LiCoO₂ and Li₂MnO₃ and spinelcompounds such as LiMn₂O₄ are known as positive electrode activematerials. Not only the behavior of a battery when those compounds areused as positive electrode active materials but also physical propertiessuch as magnetic properties, have been widely researched as disclosed inNon-Patent Documents 1 and 2, for example.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    H11-025983-   [Non-Patent Document 1] Sanghyun Lee et al., “Antiferromagnetic    ordering in Li₂MnO₃ single crystals with a two-dimensional honeycomb    lattice”, Journal of Physics: Condensed Matter, 2012, Vol. 24,    456004, pp. 1-9-   [Non-Patent Document 2] Kazuhiko Mukai et al., “Magnetic Properties    of the chemically delithiated Li_(x)Mn₂O₄ with 0.07≦x≦1”, Journal of    Solid State Chemistry, 2011, Vol. 184, issue 5, pp. 1096-1104

DISCLOSURE OF INVENTION

LiCoO₂ is used as a positive electrode active material of a lithium-ionsecondary battery. However, cobalt, a raw material of LiCoO₂, isexpensive. In view of this problem, an object is to provide a positiveelectrode active material that can be formed at low cost.

Another object is to increase the amount of lithium ions that can bereceived and released in and from a positive electrode active materialto achieve high capacity and high energy density of a secondary battery.

Furthermore, high ion conductivity and high electron conductivity arerequired as properties of a positive electrode active material of alithium-ion secondary battery. Thus, another object is to provide apositive electrode active material having high ion conductivity and highelectron conductivity. Another object is to provide a novel material.Another object is to provide a novel positive electrode active material.

Another object is to achieve high capacity and high energy density of apositive electrode of a lithium-ion secondary battery. Another object isto provide a novel battery. Another object is to provide a novellithium-ion secondary battery.

Another object is to achieve high capacity and high energy density of alithium-ion secondary battery.

Another object is to provide a highly reliable lithium-ion secondarybattery.

Note that the description of these objects does not impede the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the above objects. Other objects will be apparentfrom and can be derived from the description of the specification, thedrawings, the claims, and the like.

A lithium-manganese composite oxide (also referred to as alithium-manganese oxide) is an oxide containing at least lithium andmanganese. The lithium-manganese composite oxide may contain anothermetal, or an element such as silicon or phosphorus. In the case wherethe lithium-manganese composite oxide is used as a positive electrodematerial of a lithium-ion secondary battery, lithium may be releasedfrom the lithium-manganese composite oxide by charging.

One embodiment of the present invention is a lithium-manganese compositeoxide represented by Li_(x)Mn_(y)M_(z)O_(w), where M is a metal elementother than lithium and manganese, or silicon or phosphorus, and y, z,and w are each greater than zero and satisfy 0.26≦(y+z)/w<0.5. Thelithium-manganese composite oxide has a layered rock-salt crystalstructure.

Another embodiment of the present invention is a lithium-manganesecomposite oxide represented by Li_(x)Mn_(y)M_(z)O_(w), where M is ametal element other than lithium and manganese, or silicon orphosphorus, and y, z, and w are each greater than zero and satisfy0.26≦(y+z)/w<0.5. In the lithium-manganese composite oxide, one particleincludes a spinel crystal structure and a layered rock-salt crystalstructure in contact with the spinel crystal structure.

Another embodiment of the present invention is a lithium-manganesecomposite oxide represented by Li_(x)Mn_(y)M_(z)O_(w), where M is ametal element other than Li and Mn, or Si or P, y and z are each greaterthan zero, and x, y, z, and w satisfy 0≦x/(y+z)<2, 0.26≦(y+z)/w<0.5, and0.2<z/y<1.2.

Another embodiment of the present invention is a lithium-manganesecomposite oxide represented by Li_(x)Mn_(y)M_(z)O_(w), where M is ametal element other than Li and Mn, or Si or P, y and z are each greaterthan zero, and x, y, z, and w satisfy 0≦x/(y+z)<2 and 0.26≦(y+z)/w<0.5.The lithium-manganese composite oxide includes at least a layeredrock-salt crystal that belongs to a space group C12/m1. In the layeredrock-salt crystal, the sum of an occupancy of Mn and an occupancy of theelement represented by Mat a 2b site is greater than or equal to 40%.

In any of the above embodiments, the element represented by M ispreferably a metal element selected from Ni, Ga, Fe, Mo, In, Nb, Nd, Co,Sm, Mg, Al, Ti, Cu, and

Zn, or Si or P. Note that Ni is particularly preferable.

Another embodiment of the present invention is a lithium-manganesecomposite oxide represented by Li_(x)Mn_(y)Ni_(z)O_(w), where y and zare each greater than zero, x, y, z, and w satisfy 0≦x/(y+z)<2 and0.26≦(y+z)/w<0.5. The lithium-manganese composite oxide includes atleast a layered rock-salt crystal that belongs to a space group C12/m1.An a-axis lattice constant of the layered rock-salt crystal is largerthan or equal to 0.494 nm and a b-axis lattice constant of the layeredrock-salt crystal is larger than or equal to 0.856 nm.

Another embodiment of the present invention is a lithium-manganesecomposite oxide represented by Li_(D)Mn_(y)M_(z)O_(w), where M is ametal element other than Li and Mn, or Si or P, y and z are each greaterthan zero, and x, y, z, and w satisfy 1.35≦D/(y+z)<2 and 0.2<z/y<1.2.Furthermore, the element represented by M is preferably a metal elementselected from Ni, Ga, Fe, Mo, In, Nb, Nd, Co, Sm, Mg, Al, Ti, Cu, andZn, or Si or P.

Another embodiment of the present invention is a positive electrode inwhich a positive electrode active material layer containing any of thelithium-manganese composite oxides is over a positive electrode currentcollector.

Another embodiment of the present invention is an electric deviceincluding the positive electrode.

A positive electrode active material that can be formed at low cost canbe provided.

The amount of lithium ions that can be received and released in and froma positive electrode active material can be increased to achieve highcapacity and high energy density of a secondary battery.

High ion conductivity and high electron conductivity are required asproperties of a positive electrode active material of a lithium-ionsecondary battery. Thus, one embodiment of the present invention canprovide a positive electrode active material having high ionconductivity and high electron conductivity.

High capacity and high energy density of a lithium-ion secondary batterycan be achieved.

High capacity and high energy density of a lithium-ion secondary batterycan be achieved.

A novel material can be provided. A novel positive electrode activematerial can be provided. A novel battery can be provided. A novellithium-ion secondary battery can be provided.

The lithium-manganese composite oxide disclosed in this specificationhas high structural stability and high capacity. Furthermore, thelithium-manganese composite oxide disclosed in this specification can beformed through a simple forming process where a plurality of materialsare weighed, pulverized in a ball mill or the like, and mixed, and thenthe mixture is fired; thus, an effect of reducing cost can be obtainedand excellent mass productivity is achieved.

Firing at a high temperature of 800° C. or higher in a synthesis processof the lithium-manganese composite oxide disclosed in this specificationallows the oxide to have high crystallinity and excellent cyclecharacteristics.

Note that the description of these effects does not impede the existenceof other effects. In one embodiment of the present invention, there isno need to obtain all the above effects. Other effects will be apparentfrom and can be derived from the description of the specification, thedrawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing calculation results of the formation energy ofone embodiment of the present invention.

FIG. 2 illustrates a crystal structure of a comparative example of thepresent invention.

FIG. 3 illustrates a crystal structure of one embodiment of the presentinvention.

FIG. 4 illustrates a crystal structure of one embodiment of the presentinvention.

FIG. 5 illustrates a crystal structure of one embodiment of the presentinvention.

FIG. 6 is a graph showing calculation results of the formation energy ofone embodiment of the present invention.

FIG. 7 illustrates a crystal structure of one embodiment of the presentinvention.

FIG. 8 illustrates a crystal structure of one embodiment of the presentinvention.

FIG. 9 illustrates a crystal structure of one embodiment of the presentinvention.

FIG. 10 illustrates a crystal structure of one embodiment of the presentinvention.

FIG. 11 is a conceptual diagram illustrating a lithium-ion secondarybattery of one embodiment of the present invention at the time ofcharging.

FIG. 12 is a conceptual diagram illustrating a lithium-ion secondarybattery of one embodiment of the present invention at the time ofdischarging.

FIG. 13 is a graph showing the relationship between discharge capacityand voltage of one embodiment of the present invention and a comparativeexample.

FIG. 14 is a graph showing the relationship between discharge capacityand a ratio of compositions in one embodiment of the present invention.

FIG. 15 is a graph showing measurement results of X-ray diffraction inone embodiment of the present invention.

FIG. 16 is a graph showing the relationship between the occupancy ofatoms and a ratio of compositions in one embodiment of the presentinvention.

FIG. 17 is a graph showing the relationship between the occupancy ofatoms in one embodiment of the present invention and a ratio ofcompositions in one embodiment of the present invention.

FIG. 18 is a graph showing the relationship between a lattice constantand a ratio of compositions in one embodiment of the present invention.

FIG. 19 is a graph showing the relationship between a lattice constantand a ratio of compositions in one embodiment of the present invention.

FIG. 20 is a graph showing the relationship between a lattice constantand a ratio of compositions in one embodiment of the present invention.

FIG. 21 is a graph showing measurement results of X-ray diffraction inone embodiment of the present invention.

FIG. 22 is a graph showing measurement results of X-ray diffraction inone embodiment of the present invention.

FIG. 23 is a graph showing measurement results of X-ray diffraction inone embodiment of the present invention.

FIGS. 24A and 24B are each a graph showing a measurement result of X-raydiffraction in one embodiment of the present invention.

FIGS. 25A and 25B are each a graph showing a measurement result of X-raydiffraction in one embodiment of the present invention.

FIG. 26 is a graph showing a measurement result of X-ray diffraction inone embodiment of the present invention.

FIG. 27 is a model diagram illustrating one embodiment of the presentinvention.

FIG. 28 is a model diagram illustrating one embodiment of the presentinvention.

FIG. 29 is a cross-sectional TEM image of one embodiment of the presentinvention.

FIG. 30 is an enlarged partial image of FIG. 29.

FIG. 31 is a graph showing the relationship between discharge capacityand voltage of one embodiment of the present invention.

FIGS. 32A and 32B are model diagrams each illustrating a comparativeexample.

FIG. 33 is a graph showing the relationship between discharge capacityand voltage of one embodiment of the present invention and a comparativeexample.

FIG. 34 is a graph showing the relationship between discharge capacityand voltage of a lithium-manganese composite oxide obtained in Example.

FIG. 35 is a graph showing the relationship between discharge capacityand voltage of a secondary battery obtained in Example.

FIGS. 36A to 36C illustrate a coin-type storage battery.

FIGS. 37A and 37B illustrate a cylindrical storage battery.

FIGS. 38A and 38B each illustrate a laminated storage battery.

FIGS. 39A to 39E each illustrate a flexible laminated storage battery.

FIGS. 40A and 40B illustrate an example of a power storage device.

FIGS. 41A1, 41A2, 41B1, and 41B2 each illustrate an example of a powerstorage device.

FIGS. 42A and 42B each illustrate an example of a power storage device.

FIGS. 43A and 43B each illustrate an example of a power storage device.

FIG. 44 illustrates an example of a power storage device.

FIGS. 45A and 45B each illustrate an application example of a powerstorage device.

FIG. 46 is an external view of a storage battery.

FIG. 47 is an external view of a storage battery.

FIG. 48A illustrates electrodes of a storage battery and FIGS. 48B and48C illustrate a method for forming the storage battery.

FIGS. 49A to 49C are each a graph showing the relationship betweendischarge capacity and a ratio of compositions in one embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. The present invention is notconstrued as being limited to descriptions of the embodiments andexamples.

Embodiment 1

In this embodiment, an example of a lithium-manganese composite oxide ofone embodiment of the present invention will be described.

[1-1. Lithium-Manganese Composite Oxide]

In this embodiment, a lithium-manganese composite oxide formed bycombining LiMn_(2-A)M_(A)O₄, which is a lithium manganese oxide, havinga spinel crystal structure and Li₂Mn_(1-B)M_(B)O₃ having a layeredrock-salt (α-NaFeO₂) crystal structure is described. Note that M is ametal element other than lithium (Li) and manganese (Mn), or Si or P.

The lithium manganese composite oxide has a spinel crystal structure inpart of the surface of each particle with a layered rock-salt crystalstructure. In the case of using the lithium manganese composite oxide asa positive electrode active material of a lithium-ion secondary battery,lithium inside the particle is released or diffused through the regionwith a spinel crystal structure of the surface of the particle,resulting in high capacity. Furthermore, the lithium manganese compositeoxide preferably includes a plurality of portions each with a spinelcrystal structure such that the each particle is dotted with them. Notethat in each particle of the lithium manganese composite oxide, a regionwith a layered rock-salt crystal structure is preferably larger than theregions each with a spinel crystal structure.

Each particle contains a plurality of crystallites and the size of eachcrystallite is smaller than that of the particle, specifically, lessthan or equal to 1 μm. Note that whether the particle contains aplurality of crystallites can be determined with a high resolutiontransmission electron microscope (TEM). Furthermore, a crystal structurecan be determined with the use of a fast Fourier transformation pattern(FFT pattern) using a high resolution TEM image (multiple waveinterference image). By comparison with data on Li₂MnO₃ with a layeredrock-salt crystal structure or data on LiMn₂O₄ with a spinel structure,which is contained in a JCPDS card (database on index minerals forpowder X-ray diffraction patterns), a crystal structure can bedetermined. Thus, when some portions in the same particle of the novelmaterial are determined, at least a spot corresponding to a spinelcrystal structure and a spot corresponding to a layered rock-saltcrystal structure are observed. Note that a crystallite means thelargest aggregation that can be regarded as a single crystal and refersto a fine single crystal. The size of one crystallite can be calculated(by Scherrer formula) from peak broadening of a diffraction patternobtained using a powder X-ray diffraction method.

The lithium manganese composite oxide can also be referred to as acomposite material of crystallites of LiMn_(2-A)M_(A)O₄ (spinelcrystallites) and crystallites of Li₂Mn_(1-B)M_(B)O₃ (layered rock-saltcrystallites). FIG. 27 is a model diagram illustrating one particle ofthe lithium manganese composite oxide.

FIG. 27 illustrates that one particle includes at least two kinds ofcrystallites: a spinel crystallite 201 and a layered rock-saltcrystallite 202. As in FIG. 27, in the lithium manganese compositeoxide, one particle has a spinel crystal structure and a layeredrock-salt crystal structure in contact with the spinel crystalstructure. When a lithium battery using the lithium manganese compositeoxide as a positive electrode active material is charged or discharged,lithium of Li₂Mn_(1-B)M_(B)O₃ in each particle is released or receivedthrough the spinel crystallites 201 which are scattered on the surfaceof the particle.

Alternatively, as in a lithium-manganese composite oxide illustrated inFIG. 28, for example, the spinel crystallite 201 may be widelydistributed on a surface of one particle.

The lithium-manganese composite oxide obtained in this embodiment isrepresented by Li_(x)Mn_(y)M_(z)O_(w) (M is a metal element other thanlithium (Li) and manganese (Mn), or Si or P). In Li_(x)Mn_(y)M_(z)O_(w),the element represented by M is preferably a metal element selected fromNi, Ga, Fe, Mo, In, Nb, Nd, Co, Sm, Mg, Al, Ti, Cu, and Zn, or Si or P,and Ni is the most preferable. Note that the number of kinds of elementselected as M is not necessarily one and may be two or more.

[1-2. Synthesis of Lithium-Manganese Composite Oxide]

A synthesis method of the lithium-manganese composite oxide representedby Li_(x)Mn_(y)M_(z)O_(w) is described in detail below. Here, Ni is usedas the element M.

As raw materials of the lithium-manganese composite oxide, Li₂CO₃,MnCO₃, and NiO can be used, for example.

First, each of the raw materials is weighed to have the desired molarratio.

Next, acetone is added to the powder of these materials, and then, theyare mixed in a ball mill to prepare mixed powder.

After that, heating is performed to volatilize acetone, so that a mixedmaterial is obtained.

Then, the mixed material is put into a crucible, and is subjected tofirst firing at temperatures higher than or equal to 800° C. and lowerthan or equal to 1100° C. in the air for 5 to 20 hours inclusive tosynthesis a novel material.

Subsequently, grinding is performed to separate the sintered particles.For the grinding, acetone is added and then mixing is performed in aball mill.

After the grinding, heating is performed to volatilize acetone, andthen, vacuum drying is performed, so that a powdery novel material isobtained.

To increase the crystallinity or to stabilize the crystal, second firingmay be performed after the first firing. The second firing is performedat temperatures higher than or equal to 500° C. and lower than or equalto 800° C., for example.

The second firing may be performed in a nitrogen atmosphere, forexample.

Although Li₂CO₃, MnCO₃, and NiO are used as starting materials in thisembodiment, the materials are not limited thereto and can be othermaterials.

When the ratio for weighing (also referred to as the feed ratio of rawmaterials) is changed, for example, a composite oxide with a layeredrock-salt crystal structure and a spinel crystal structure can beobtained.

The ratio for weighing is the molar ratio between the raw materialsused. For example, in the case where raw materials in whichLi₂CO₃:MnCO₃:NiO=1:1.5:0.5 are used, the ratio of MnCO₃ to NiO is 3(MnCO₃/NiO=1.5÷0.5). Note that the term “Ni/Mn (feed ratio of rawmaterials)” or “raw material feed ratio of Ni to Mn”, for example,explains the molar ratio of Ni to Mn among raw materials used. In thecase where raw materials in which Li₂CO₃:MnCO₃:NiO=1:1.5:0.5 are used,for example, Li/Ni is 4 (Li/Mn=(1×2)÷0.5), whereas Mn/Ni is 3(Mn/Ni=1.5÷0.5).

Here, the idea of changing the ratio for weighing is described.

In LiMn₂O₄ with a spinel structure, the atomic ratio of Li to Mn is 1:2,whereas in Li₂MnO₃ with a layered rock-salt structure, the atomic ratioof Li to Mn is 2:1. Thus, when the ratio of Mn to Li is made larger than1/2, the proportion of the spinel structure can be increased, forexample.

Here, described is the case where Li₂CO₃ and MnCO₃ are used as startingmaterials so that the spinel crystallites 201 are included atapproximately 2%.

Li₂CO₃ and MnCO₃ are weighed to have the ratio of 0.98:1.01, pulverizedin a ball mill or the like, and fired at temperatures higher than orequal to 800° C. and lower than or equal to 1100° C.

Note that “the spinel crystallites 201 are included at approximately 2%”means that the layered rock-salt crystallites 202 are included atapproximately 98%.

In the case where each particle includes the spinel crystallites 201 atapproximately 5%, Li₂CO₃ and MnCO₃ are weighed so that the ratio ofLi₂CO₃ to MnCO₃ is 0.955:1.03, and they are pulverized in a ball mill orthe like and fired.

In the case where each particle includes the spinel crystallites 201 atapproximately 50%, Li₂CO₃ and MnCO₃ are weighed so that the ratio ofLi₂CO₃ to MnCO₃ is 0.64:1.28, and they are pulverized in a ball mill orthe like and fired.

The novel material is formed by intentionally changing the feed ratio ofraw materials so that the spinel crystallites 201 are included atgreater than or equal to approximately 2% and less than or equal to 50%.

The above is the idea of changing the ratio for weighing.

Note that even in the case where raw materials are weighed so that thespinel crystallites are included at a predetermined proportion, theproportion of the spinel crystallites in an actually synthesizedlithium-manganese composite oxide might be different from thepredetermined proportion in some cases. As will be described in detailin Example, it is suggested that the lithium-manganese composite oxideof one embodiment of the present invention has a structure in which Mnis substituted at some of Li sites in LiMnO₃. Accordingly, in the casewhere the feed ratio is changed, that is, in the case where the ratio ofMn to Li is increased, the increased Mn might be used for both of theformation of spinel crystallites and the substitution of Mn at some Lisites in LiMnO₃ with a layered rock-salt structure.

Note that although the case where Ni is not contained is described herefor easy understanding, the same applies to the case where Ni iscontained.

The feed ratio is changed to form a lithium manganese composite oxidehaving a spinel crystal structure in part of the surface of eachparticle with a layered rock-salt crystal structure.

FIG. 29 is a cross-sectional TEM image of the novel material obtained inthis embodiment.

FIG. 30 is an enlarged image of one of a plurality of particles in FIG.29. As shown in FIG. 30, a region surrounded by a black dotted linecorresponds to the spinel crystallite 201, and a region surrounded by awhite dotted line corresponds to the layered rock-salt crystallite 202.

When an FFT pattern is obtained by the FFT analysis from a part of theregion surrounded by the black dotted line in FIG. 30, the valuesdetermined by the positional relationship (e.g., a distance and anangle) of the obtained spots correspond to the data of JCPDS card (e.g.,an incident angle and a diffraction intensity) on LiMn₂O₄ with a spinelcrystal structure. Thus, the region can be identified to have a spinelcrystal structure.

Furthermore, the positional relationship (e.g., a distance and an angle)of spots in an FFT pattern obtained by the FFT analysis from a part ofthe region surrounded by the white dotted line in FIG. 30 and that inthe data contained in a JCPDS card (e.g., an incident angle and adiffraction intensity) on Li₂MnO₃ with a layered rock-salt crystalstructure are compared. As a result, the region can be identified tohave a layered rock-salt crystal structure.

FIG. 31 shows the discharge capacity of the obtained lithium-manganesecomposite oxide. The vertical axis represents voltage (V), and thehorizontal axis represents discharge capacity (mAh/g). The plot denotedby “Novel material” shows the discharge capacity of thelithium-manganese composite oxide obtained in this embodiment.

FIG. 32A illustrates a comparative example 1, and FIG. 32B illustrates acomparative example 2. The novel material is greatly different from thecomparative examples 1 and 2 in structure and property. FIG. 32Aillustrates a mixture of a plurality of particles, that is, a mixture ofparticles each having a spinel crystal structure and a size of severalmicrometers (Spi-LiMn₂O₄ particles 204) and Li₂MnO₃ particles 203 eachhaving a layered rock-salt crystal structure and a size of severalmicrometers. FIG. 32B illustrates a material 205 obtained by sinteringthe mixture in FIG. 32A at high temperatures (e.g., 1000° C.). Inobtaining the Li₂MnO₃ particles 203 from the comparative example 1 or 2,Li₂CO₃ (lithium carbonate) and MnCO₃ (manganese carbonate) are weighedso that the ratio of Li₂CO₃ to MnCO₃ is 1:1, and they are pulverized ina ball mill or the like and fired. In obtaining the Spi-LiMn₂O₄particles 204, Li₂CO₃ and MnCO₃ are weighed so that the ratio of Li₂CO₃to MnCO₃ is 0.5:2, and they are pulverized in a ball mill or the likeand fired.

The comparative example 1, which corresponds to FIG. 32A, is a sampleformed in such a manner that particles each having a size of severalmicrometers and a spinel crystal structure (Spi-LiMn₂O₄ particles 204)and the Li₂MnO₃ particles 203 each having a size of several micrometersand a layered rock-salt crystal structure are separately synthesized andthe Spi-LiMn₂O₄ particles 204 and the Li₂MnO₃ particles 203 are mixed.

The comparative example 2, which corresponds to FIG. 32B, is a sampleobtained by firing the comparative sample 1 at 1000° C.

As shown in FIG. 31, the discharge capacity of the obtainedlithium-manganese composite oxide is superior to those of thecomparative examples 1 and 2. Thus, the obtained lithium-manganesecomposite oxide having a spinel crystal structure in part of the surfaceof each layered rock-salt particle has a high capacity.

Note that the layered rock-salt crystal structure represented by thegeneral formula Li₂MO₃ (M is a metal element, Si, or P) includes one Matom and two Li atoms; thus, if every Li contributes to charge anddischarge, the capacity higher than that of the spinel crystal structurerepresented by the general formula LiM₂O₄ (M is a metal element, Si, orP) can be obtained. However, as shown in FIG. 31, the discharge capacityof the composite oxide of one embodiment of the present invention ismuch higher than that of a mixture of LiM₂O₄ and Li₂MO₃, such as thecomparative example 1 or the comparative example 2. However, when toomany spinel crystal structures with low capacity are mixed, the capacityof the obtained lithium-manganese composite oxide becomes low. Thus, itis important to synthesize a lithium-manganese composite oxide usingspinel crystal structures as few as possible to obtain high capacity.

In the case where 100% of the lithium-manganese composite oxideLi_(x)Mn_(y)M_(z)O_(w) (M is a metal element other than lithium (Li) andmanganese (Mn), or Si or P) is LiMn_(y)M_(z)O₄ (M is a metal elementother than lithium (Li) and manganese (Mn), or Si or P) having a spinelcrystal structure, x=1 and (y+z)/w=0.5 are satisfied, whereas in thecase where 100% of Li_(x)Mn_(y)M_(z)O_(w) is Li₂MnO₃ having a layeredrock-salt crystal structure, x=2 and (y+z)/w=0.333 are satisfied. Here,x changes because of charge and discharge; x is increased when lithiumis released from a positive electrode and is decreased when lithium isinserted. Note that because the above relationships of x, y, z, and ware satisfied in the ideal case, the proportions of the elements mightfluctuate by approximately 20% when either transition metals or oxygenis reduced during synthesis, for example. For this reason, x≦2.2, y>0,z>0, w>0, and 0.26≦(y+z)/w<0.5 are satisfied. Since the proportion ofthe spinel crystal structures is preferably small, it is preferable that0.3 (y+z)/w≦0.45 be also satisfied.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 2

In this embodiment, a lithium-manganese composite oxide of oneembodiment of the present invention will be described.

[2-1. Lithium-Manganese Composite Oxide Li_(x)Mn_(y)M_(z)O_(w)]

The inventors have found that high capacity can be obtained when alithium-manganese composite oxide represented by Li_(x)Mn_(y)M_(z)O_(w)is used as a positive electrode material of a lithium-ion secondarybattery. In addition, it has been suggested that the capacity depends onthe ratio of Mn to the element M, and that high capacity can be obtainedwhen the ratio is within a particular range.

The following describes the element M, and x, y, z, and w in thelithium-manganese composite oxide represented by Li_(x)Mn_(y)M_(z)O_(w)of one embodiment of the present invention. The element M is preferablya metal element other than lithium and manganese, or silicon orphosphorus. Furthermore, 0≦x/(y+z)<2, y>0, z>0, and 0.26≦(y+z)/w<0.5 arepreferably satisfied. In addition, the ratio of the element M to Mn(z/y) is preferably greater than 0.2 and less than 1.2, furtherpreferably greater than 0.2 and less than or equal to 0.9, still furtherpreferably greater than or equal to 0.25 and less than or equal to 0.6.

In the case where Ni is used as the element M in the lithium-manganesecomposite oxide Li_(x)Mn_(y)M_(z)O_(w) described in this embodiment, theratio of Ni to Mn (Ni/Mn) is set to greater than or equal to 0.276, sothat the capacity of a battery in which the lithium-manganese compositeoxide is used for a positive electrode active material layer included ina positive electrode can be increased. Note that the ratio of Ni to Mn(Ni/Mn) is the feed ratio of raw materials.

In the lithium-manganese composite oxide Li_(x)Mn_(y)M_(z)O_(w), Mn andM are substituted at some of the Li sites. In the case where Ni is usedas the element M and the ratio of Ni to Mn (Ni/Mn, which is the feedratio of raw materials) is greater than 0.2, preferably greater than orequal to 0.276, the 2b site, the 2c site, and the 4h site are occupiedby Ni and Mn. Furthermore, the sum of the occupancy of Ni and theoccupancy of Mn at the 2b site is greater than or equal to 40%,preferably greater than or equal to 40% and less than or equal to 85%,further preferably greater than or equal to 40% and less than or equalto 75%. In addition, the sum of the occupancy of Ni at the 2c site andthe 4h site and the occupancy of Mn at the 2c site and the 4h site isgreater than or equal to 0.2%, preferably greater than or equal to 0.5%.

When at least one of the 2b site, the 2c site, and the 4h site isoccupied by Ni or Mn, crystal distortion, a change in electron state, orthe like occurs; thus, Li is easily diffused. As a result, the capacityof the battery in which the lithium-manganese composite oxide is usedfor the positive electrode active material layer included in thepositive electrode can be increased.

Although Ni is described as a typical example of M in thelithium-manganese composite oxide Li_(x)Mn_(y)M_(z)O_(w) here, anothermetal element other than lithium and manganese, or silicon or phosphoruscan be used as appropriate to obtain the same effect.

[2-2. X-Ray Diffraction]

Next, a crystal structure is described which is identified by X-raydiffraction (XRD) measurement performed on an example of alithium-manganese composite oxide Li_(x)Mn_(y)M_(z)O_(w) where Ni isused as M and the ratio of Ni to Mn (Ni/Mn, which is the feed ratio ofraw materials) is greater than or equal to 0.276.

[2-3. Rietveld Analysis 1]

The crystal structure data of the lithium-manganese composite oxide canbe acquired by the Rietveld analysis. As analysis software, TOPAS(DIFFRAC^(plus) TOPAS Version 3) manufactured by Bruker AXS is used. Onthe assumption that the obtained lithium-manganese composite oxide hasthe first phase and the second phase, the Rietveld analysis is performedon the basis of the X-ray diffraction measurement. Fitting is performedunder the conditions where the initial first phase is Li₂MnO₃ with alayered rock-salt structure that belongs to the space group C12/m1 andthe initial second phase is LiMn₂O₄ with a spinel structure that belongsto the space group Fd-3m. Table 1 shows the crystal data of Li₂MnO₃ witha layered rock-salt structure (C12/m1). Table 2 shows the crystal dataof LiMn₂O₄ with a spinel structure (Fd-3m). Data in Table 1 is citedfrom Non-Patent Document 1. Data in Table 2 is cited from TOPAS(DIFFRAC^(plus) TOPAS Version 3) database and thus is the same ascrystal data described in Non-Patent Document 2 although the expressionis slightly different. Note that the coordinates might be changed fromthe initial coordinates by the fitting; however, the change does notgreatly affect the symmetry. Note that here, for the calculation,“preferred orientation” was set and the algorithm of spherical harmonicswas used.

TABLE 1 Layered rock-salt structure: Li₂MnO₃ Space group: C2/m (No. 12)a(Å) = 4.9167, b(Å) = 8.5069, c(Å) = 5.0099 β(degree) = 109.373 atomsite x y z B[Å²] Mn 4g 0 0.1663 0 0.73 Li1 2b 0 0.5 0 0.97 Li2 2c 0 00.5 0.97 Li3 4h 0 0.6560 0 0.97 O1 4i 0.2178 0 0.2253 0.64 O2 8i 0.25370.3220 0.2237 0.64

TABLE 2 Spinel structure: LiMn₂O₄ Space group: Fd-3m (No. 227) a(Å) =8.2404 atom site x y z B Mn 16c 0 0 0 0.73 Li  8b 0.3750 0.3750 0.37500.97 O 32e 0.2380 0.2380 0.2380 0.64

In Tables 1 and 2, B denotes a temperature factor called theDebye-Waller factor. The weight proportion of the layered rock-saltstructure that belongs to the space group C12/m1 to the spinel structurethat belongs to the space group Fd-3m was calculated by the Rietveldanalysis; accordingly, the weight proportion of the spinel structure isless than or equal to approximately 1.1% when the feed ratio of rawmaterials Ni/Mn is greater than or equal to 0.276.

[2-4. Rietveld Analysis 2]

Next, for more detailed examination of the occupancies of atoms at eachsite in a layered rock-salt crystal structure, calculation is performedfor the occupancies of Li, Mn, and Ni at three sites: the 2b site, the2c site, and the 4h site. Note that as shown in Table 1, in Li₂MnO₃ witha layered rock-salt structure that belongs to the space group C12/m1,the 2b site, the 2c site, and the 4h site are occupied by Li. Here, tocalculate the occupancies of the atoms, the Rietveld analysis isperformed on the assumption that the obtained lithium-manganesecomposite oxide Li_(x)Mn_(y)M_(z)O_(w) is a single layer of a layeredrock-salt structure. Note that the occupancy is the probability ofexistence of an atom at a given site.

Because of the small difference of the X-ray scattering power between Niand Mn, Ni and Mn are hard to distinguish. For this reason, the sum ofthe occupancy of Ni and the occupancy of Mn is discussed here.

The occupancies of an element X at the 2b site, the 2c site, and the 4hsite are represented by A(X)_(2b), A(X)_(2c), and A(X)_(4h). Note thatthe 2c site and the 4h site of Li₂MnO₃ are roughly distributed in alayered manner. Thus, the sum of the occupancies at the 4h site and the2c site is calculated here. Because the number of the 4h sites are twicethe number of the 2c sites, the occupancy A(X)_(2c+4h), which is the sumof the occupancies at the 4h site and the 2c site, is defined by Formula(1).

A(X)_(2c+4h) =[A(X)_(2c)×1+A(X)_(4h)×2]÷(1+2)  (1)

The occupancy of the element M at the 2b site can be represented byA(M)_(2b), for example. Furthermore, the occupancy of Mn at the 2b sitecan be represented by A(Mn)_(2b).

Here, when the feed ratio of raw materials (Ni/Mn) is greater than orequal to 0.276, the sum of A(Ni)_(2b) and A(Mn)_(2b),[A(Ni)_(2b)+A(Mn)_(2b)], is greater than or equal to 57.8%, and the sumof A(Ni)_(2c+4h) and A(Mn)_(2c+4h), [A(Ni)_(2c+4h)+A(Mn)_(2c+4h)], isgreater than or equal to approximately 1%.

[2-5. Calculation of Formation Energy]

In Li₂MnO₃ with a layered rock-salt structure that belongs to the spacegroup C12/m1, Ni is substituted at each atomic site, and the electronstate and the formation energy of the Ni substitution are calculated.The formation energy of the Ni substitution is calculated by using thefollowing Formula (2).

Li₂MnO₃ +xNiO→Li_(2-x)Ni_(x)MnO₃ +xLiCO₃  (2)

The cohesive energies of Li₂MnO₃, NiO, Li_(2-x)Ni_(x)MnO₃, and LiCO₃ arerepresented by E_(a), E_(b), E_(c), and E_(d), respectively, theformation energy E_(form) is represented by the following Formula (3).

E _(form) =E _(a) +E _(b)−(E _(a) +E _(d))  (3)

The electron state and the formation energy were numerically measuredunder the conditions shown in Table 3. The Vienna Ab initio SimulationPackage (VASP) was used for the measurement of the electron state.

TABLE 3 Pseudopotential/Functional PAW/HSE06 Cut-off energy 800 eVSampling width of k-points ka 0.10 [/Å] kb 0.12 [/Å] kc 0.10 [/Å] Spinpolarization setup

The supercell used for the calculation includes 32 Li atoms, 16 Mnatoms, and 48 O atoms. Note that the supercell is a crystal latticewhich is obtained by repeating the unit lattice (unit cell) naturalnumber of times in the crystal axis directions and by being defined as acrystal periodic unit. Here, the supercell including 2×1×2 unit lattices(unit cells) is used. The unit lattice (unit cell) of Li₂MnO₃ includes 8Li atoms, 4 Mn atoms, and 12 O atoms. Note that a k-point is a latticepoint in a reciprocal lattice space, and the sampling width of thek-points is the distance between the k-points used for sampling in areciprocal lattice space.

[2-5. A. One-Site Substitution]

In Condition (A1), Ni is substituted at any of the 2b site, the 2c site,and the 4h site. Condition (A1) is represented by the following chemicalformula (Formula (4)).

Li₃₂₋₁Mn₁₆NiO₄₈  (4)

In Condition (A2), Ni is substituted at the 4g site. Condition (A2) isrepresented by the following chemical formula (Formula (5)).

Li₃₂Mn₁₆₋₁NiO₄₈  (5)

FIG. 2 illustrates a crystal structure of Li₂MnO₃ before Nisubstitution. FIG. 1 shows the calculation results of the formationenergy under each of Conditions (A1) and (A2). FIG. 1 indicates that theminimum formation energy under Condition (A1) is smaller than that underCondition (A2). Thus, it is assumed that Ni is more likely to besubstituted at the 2b site, the 2c site, and the 4h site than at the 4gsite, where a Mn atom exists in Li₂MnO₃.

FIGS. 3 to 5 illustrate crystal structures corresponding to thecalculation results in FIG. 1. FIGS. 3 and 4 each illustrate an exampleof atomic arrangement under Condition (A1). FIG. 3 illustrates anexample of the atomic arrangement with the highest formation energyunder Condition (A1) shown in FIG. 1, which is the case where Ni issubstituted at the 4h site. FIG. 4 illustrates an example of the atomicarrangement with the lowest formation energy under Condition (A1), whichis the case where Ni is substituted at the 2b site.

FIG. 5 illustrates an example of atomic arrangement under Condition(A2).

[2-5. B. Two-Site Substitution]

In Condition (B1), two Ni atoms are substituted at any of the 2b site,the 2c site, and the 4h site. Condition (B1) is represented by thefollowing chemical formula (Formula (6)).

Li₃₂₋₂Mn₁₆Ni₂O₄₈  (6)

In Condition (B2), one Ni atom is substituted at any of the 2b site, the2c site, and the 4h site and another Ni atom is substituted at the 4gsite. Condition (B2) is represented by the following chemical formula(Formula (7)).

Li₃₂₋₁Mn₁₆₋₁Ni₂O₄₈  (7)

In Condition (B3), two Ni atoms are substituted at the 4g sites.Condition (B3) is represented by the following chemical formula (Formula(8)).

Li₃₂Mn₁₆₋₂Ni₂O₄₈  (8)

FIG. 6 shows the calculation results of the formation energy under eachof Conditions (B1), (B2), and (B3). The formation energy under Condition(B3), that is, the formation energy of Ni substitution at the 4g site,is very high. In contrast, under Condition (B2), which is the case whereone Ni atom is substituted at any of the 2b site, the 2c site, and the4h site and another Ni atom is substituted at the 4g site, the formationenergy of Ni substitution is low.

FIGS. 7 to 10 illustrate crystal structures corresponding to thecalculation results in FIG. 6. Note that each of the crystal structuresillustrated in FIGS. 7 to 10 is little larger than one unit of thesupercell used for the calculation; although the supercell includes 32Li atoms, 16 Mn atoms, and 48 O atoms as one unit, more atoms areillustrated in each of FIGS. 7 to 10.

FIGS. 7 and 8 each illustrate an example of atomic arrangement underCondition (B1). FIG. 7 illustrates an example of the atomic arrangementwith the highest formation energy under Condition (B1) shown in FIG. 6,which is the case where one Ni atom is substituted at the 2b site andanother Ni atom is substituted at the 2c site. FIG. 8 illustrates anexample of the atomic arrangement with the lowest formation energy underCondition (B1), which is the case where two Ni atoms are substituted atthe 2b sites. In FIG. 8, the substituted atoms are arranged close toeach other.

FIGS. 9 and 10 are each an example of atomic arrangement under Condition(B2). FIG. 9 illustrates an example of the atomic arrangement with thehighest formation energy under Condition (B2) shown in FIG. 6, which isthe case where one Ni is substituted at the 2b site and another Ni atomis substituted at the 4g site.

FIG. 10 illustrates the atomic arrangement with the lowest formationenergy under Condition (B2) shown in FIG. 6. FIG. 10 illustrates thecase of Condition (B2) as in FIG. 9, in which one Ni atom is substitutedat the 2b site and another Ni atom is substituted at the 4g site; FIG.10 is different from FIG. 9 in that the Ni atoms are arranged in thesame layer.

The above results suggest that the following applies to the case of Nisubstitution in Li₂MnO₃. In the case of one-site substitution, theformation energy is low when a Ni atom is substituted at the 2b site ofLi sites in Li₂MnO₃, the 2b site, the 2c site, and the 4h site. In thecase of two-site substitution, the formation energy is low when one Niatom is substituted at any of Li sites in Li₂MnO₃, the 2b site, the 2csite, and the 4h site, and another Ni atom is substituted at the 4gsite, which is a Mn site in Li₂MnO₃.

Thus, also from the calculation of the formation energy, it is suggestedthat a Ni atom is likely to be substituted at any of the 2b site, the 2csite, and the 4h site, which are Li sites. In addition, the Rietveldanalysis based on the X-ray diffraction suggests that a Ni atom or a Mnatom is possibly substituted at any of the 2b site, the 2c site, and the4h site, which are Li sites. Accordingly, a Ni atom or a Mn atomprobably occupies at least one of the 2b site, the 2c site, and the 4hsite in the obtained lithium-manganese composite oxide.

When the lithium-manganese composite oxide described in this embodimentis used for a lithium-ion secondary battery, the battery can have highcapacity and high energy density.

[2-6. Synthesis of Lithium-Manganese Composite Oxide]

A synthesis method of the lithium-manganese composite oxide representedby Li_(x)Mn_(y)M_(z)O_(w) is described in detail below. Here, Ni is usedas the element M.

In this embodiment, the ratio for weighing is varied to obtain thelithium-manganese composite oxide of one embodiment of the presentinvention.

For easy understanding, the case where z=0, that is, the case of no Ni,is described first. In order to form Li₂MnO₃ with a layered rock-saltstructure, which is a comparative sample, the molar ratio of lithium tomanganese is set to 2:1, whereas in order to form the lithium-manganesecomposite oxide of one embodiment of the present invention, the ratio oflithium to manganese is decreased, for example.

Here, the case where z>0 is described. In that case, when the molarratio of lithium to manganese in the case where z=0 is set to 1.68:1.12,some Mn atoms are substituted with Ni atoms. When the molar ratio of Mnto Ni is 0.8062:0.318, for example, the molar ratio of Li to Mn and Niis set to 1.68:0.8062:0.318. In the case where Li₂CO₃, MnCO₃, and NiOare used as starting materials, the starting materials are weighed sothat the molar ratio of Li₂CO₃ to MnCO₃ and NiO is 0.84:0.8062:0.318.

Next, acetone is added to the powder of these materials, and then, theyare mixed in a ball mill to prepare mixed powder.

After that, heating is performed to volatilize acetone, so that a mixedmaterial is obtained.

Then, the mixed material is put into a crucible, and is subjected tofirst firing at temperatures higher than or equal to 800° C. and lowerthan or equal to 1100° C. in the air for 5 to 20 hours inclusive tosynthesis a novel material.

Subsequently, grinding is performed to separate the sintered particles.For the grinding, acetone is added and then mixing is performed in aball mill.

After the grinding, heating is performed to volatilize acetone, andthen, vacuum drying is performed, so that a powdery novel material isobtained.

To further increase the capacity of the obtained Li_(x)Mn_(y)M_(z)O_(w),the ratio of the element M to Mn (M/Mn, which is the feed ratio of rawmaterials) is preferably greater than 0.2 and less than 1.2, furtherpreferably greater than 0.2 and less than or equal to 0.9, still furtherpreferably greater than or equal to 0.25 and less than or equal to 0.6.

To increase the crystallinity or to stabilize the crystal, second firingmay be performed after the first firing. The second firing is performedat temperatures higher than or equal to 500° C. and lower than or equalto 800° C., for example.

The second firing may be performed in a nitrogen atmosphere, forexample.

Although Li₂CO₃, MnCO₃, and NiO are used as starting materials in thisembodiment, the materials are not limited thereto and can be othermaterials.

Accordingly, the obtained lithium-manganese composite oxide can be usedas a positive electrode active material to form a favorable positiveelectrode.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 3

This embodiment shows an example in which Li₂Mn_(y)M_(1-y)O₃ (M is ametal element other than lithium (Li) and manganese (Mn), or Si or P)with a layered rock-salt (α-NaFeO₂) crystal structure is used as apositive electrode active material.

A synthesis method of Li₂Mn_(y)M_(1-y)O₃ is described below. In thisembodiment, the case where y=0.9 is described as an example. Table 4shows raw materials for forming a comparative sample 150 and samples 151to 166. In this embodiment, the comparative sample 150 and the samples151 to 166 are formed by combination of the raw materials shown in Table4. Note that “Li raw material”, “Mn raw material”, and “M raw material”in Table 4 are, for example, raw materials containing Li, Mn, and M,respectively. When M raw materials containing different elements Mareused, various kinds of Li₂Mn_(y)M_(1-y)O₃ containing different elementsM can be synthesized, for example.

TABLE 4 Li Material Mn Material M Material Sample 150 Li₂CO₃ MnCO₃ —Sample 151 Li₂CO₃ MnCO₃ NiO Sample 152 Li₂CO₃ MnCO₃ Ga₂O₃ Sample 153Li₂CO₃ MnCO₃ FeC₂O₄ Sample 154 Li₂CO₃ MnCO₃ MoO₃ Sample 155 Li₂CO₃ MnCO₃In₂O₃ Sample 156 Li₂CO₃ MnCO₃ Nb₂O₅ Sample 157 Li₂CO₃ MnCO₃ Nd₂O₃ Sample158 Li₂CO₃ MnCO₃ Co₃O₄ Sample 159 Li₂CO₃ MnCO₃ Sm₂O₃ Sample 160 Li₂CO₃MnCO₃ NH₄H₂PO₄ Sample 161 Li₂CO₃ MnCO₃ MgO Sample 162 Li₂CO₃ MnCO₃ SiO₂Sample 163 Li₂CO₃ MnCO₃ Al₂O₃ Sample 164 Li₂CO₃ MnCO₃ Ti₂O₃ Sample 165Li₂CO₃ MnCO₃ CuO Sample 166 Li₂CO₃ MnCO₃ ZnO

First, the materials shown as the Li raw materials, the Mn rawmaterials, and the M raw materials in Table 4 are weighed. In thisembodiment, samples in each of which y is 0.9 are formed. This meansthat each of the samples is formed so that the raw material feed ratio(molar ratio) of Li to Mn and M is adjusted to 2:0.9:0.1. In the case offorming the sample 151, for example, raw materials are weighed so thatthe molar ratio of lithium carbonate (Li₂CO₃) to manganese carbonate(MnCO₃) and nickel oxide (NiO) is set to be 1:0.9:0.1. In the case offorming the sample 152, raw materials are weighed so that the molarratio of Li₂CO₃ to MnCO₃ and gallium oxide (Ga₂O₃) is set to 1:0.9:0.05.The comparative sample 150 and the samples 151 to 166 are formed in thesame manner except for the feed ratios of raw materials.

Next, acetone is added to the raw materials, and then, the raw materialsare mixed in a ball mill to form a mixed material. In this embodiment,the weighed materials, a zirconia ball with a diameter of 3 mm, andacetone are put into a pot made of zirconia, and wet ball milling usinga planetary ball mill is performed at 400 rpm for 2 hours.

After that, heating is performed to volatilize acetone, so that a mixedmaterial is obtained. In this embodiment, acetone in slurry subjected tothe ball milling is volatilized at 50° C. in the air to obtain the mixedmaterial.

Then, the mixed material is put into a melting pot, and is fired attemperatures in the range from 500° C. to 1000° C. in the air for 5 to20 hours inclusive to synthesize a novel material. In this embodiment,an alumina melting pot is filled with the mixed material that has beendried, and heating is performed at 900° C. for 10 hours.

Subsequently, grinding is performed to separate the sintered particles.In this embodiment, the fired material, a zirconia ball with a diameterof 3 mm, and acetone are put into a pot made of zirconia, and wet ballmilling using a planetary ball mill is performed at 200 rpm for 2 hours.

After the grinding, heating is performed to volatilize acetone, andthen, vacuum drying is performed, so that powdery novel materials areobtained. In this embodiment, heating is performed on the mixturesubjected to wet ball milling at 50° C. in the air to volatilizeacetone, and then, vacuum drying is performed at 170° C.

FIG. 33 shows the measurement results of the discharge capacity of eachof the comparative sample 150 and the samples 151 to 166. An enlargedview of a part 170 in FIG. 33 is illustrated in the upper right portionof the same drawing.

As illustrated in FIG. 33, each of the samples 151 and 166 has dischargecapacity higher than that of the comparative sample 150. In particular,the sample 151 in which Ni is used as M has the highest dischargecapacity.

Thus, the novel material (any of the samples 151 to 166) can be used asa positive electrode active material to form a favorable positiveelectrode.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 4

In this embodiment, the structure of a storage battery including thepositive electrode active material formed by the forming methoddescribed in Embodiment 1 will be described with reference to FIGS. 36Ato 36C, FIGS. 37A and 37B, and FIGS. 38A and 38B.

[Coin-Type Storage Battery]

FIG. 36A is an external view of a coin-type (single-layer flat type)storage battery, and FIG. 36B is a cross-sectional view thereof

In a coin-type storage battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. The positiveelectrode active material layer 306 may further include a binder forincreasing adhesion of positive electrode active materials, a conductiveadditive for increasing the conductivity of the positive electrodeactive material layer, and the like in addition to the active materials.As the conductive additive, a material that has a large specific surfacearea is preferably used; for example, acetylene black (AB) can be used.Alternatively, a carbon material such as a carbon nanotube, graphene, orfullerene can be used.

A negative electrode 307 includes a negative electrode current collector308 and a negative electrode active material layer 309 provided incontact with the negative electrode current collector 308. The negativeelectrode active material layer 309 may further include a binder forincreasing adhesion of negative electrode active materials, a conductiveadditive for increasing the conductivity of the negative electrodeactive material layer, and the like in addition to the negativeelectrode active materials. A separator 310 and an electrolyte (notillustrated) are provided between the positive electrode active materiallayer 306 and the negative electrode active material layer 309.

A material with which lithium can be dissolved and precipitated or amaterial into and from which lithium ions can be inserted and extractedcan be used for the negative electrode active materials used for thenegative electrode active material layer 309; for example, a lithiummetal, a carbon-based material, and an alloy-based material can be used.The lithium metal is preferable because of its low redox potential(3.045 V lower than that of a standard hydrogen electrode) and highspecific capacity per unit weight and per unit volume (3860 mAh/g and2062 mAh/cm³).

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, carbon black, and the like.

Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, or pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal (higher than or equal to 0.1 V and lower than or equal to 0.3 Vvs. Li/Li⁺) when lithium ions are intercalated into the graphite (whilea lithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage. Inaddition, graphite is preferable because of its advantages such asrelatively high capacity per unit volume, small volume expansion, lowcost, and safety greater than that of a lithium metal.

For the negative electrode active materials, an alloy-based materialwhich enables charge-discharge reactions by an alloying reaction and adealloying reaction with lithium can be used. A material containing atleast one of Al, Si, Ge, Sn, Pb, Sb, Bi, Ag, Au, Zn, Cd, In, Ga, and thelike can be used, for example. Such elements have higher capacity thancarbon. In particular, silicon has a significantly high theoreticalcapacity of 4200 mAh/g. For this reason, silicon is preferably used forthe negative electrode active materials. Examples of the alloy-basedmaterial using such elements include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂,Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like. Here, SiO is amaterial that contains silicon at higher proportion than SiO₂ does.

Alternatively, for the negative electrode active materials, an oxidesuch as titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), and molybdenum oxide (MoO₂) can be used.

Still alternatively, for the negative electrode active materials,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li₂₆Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive materials and thus the negative electrode active materials can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedfor the negative electrode active materials; for example, a transitionmetal oxide which does not cause an alloy reaction with lithium, such ascobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may beused. Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃.

The current collectors 305 and 308 can each be formed using a highlyconductive material which is not alloyed with a carrier ion of lithiumamong other elements, such as a metal typified by stainless steel, gold,platinum, zinc, iron, nickel, copper, aluminum, titanium, and tantalumor an alloy thereof. Alternatively, an aluminum alloy to which anelement which improves heat resistance, such as silicon, titanium,neodymium, scandium, and molybdenum, is added can be used. Stillalternatively, a metal element which forms silicide by reacting withsilicon can be used. Examples of the metal element which forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, andthe like. The current collectors can each have a foil-like shape, aplate-like shape (sheet-like shape), a net-like shape, a cylindricalshape, a coil shape, a punching-metal shape, an expanded-metal shape, orthe like as appropriate. The current collectors each preferably have athickness of 10 μm to 30 μm inclusive.

Any of the positive electrode active materials described in Embodiment 1can be used for the positive electrode active material layer 306.

As the separator 310, an insulator such as cellulose (paper),polyethylene with pores, and polypropylene with pores can be used.

As an electrolyte in the electrolytic solution, a material whichcontains carrier ions is used. Typical examples of the electrolyte arelithium salts such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiCF₃SO₃,Li(CF₃SO₂)₂N, and Li(C₂F₅SO₂)₂N. One of these electrolytes may be usedalone, or two or more of them may be used in an appropriate combinationand in an appropriate ratio.

Note that when carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions,instead of lithium in the above lithium salts, an alkali metal (e.g.,sodium and potassium), an alkaline-earth metal (e.g., calcium,strontium, barium, beryllium, and magnesium) may be used for theelectrolyte.

As a solvent of the electrolytic solution, a material in which carrierions can transfer is used. As the solvent of the electrolytic solution,an aprotic organic solvent is preferably used. Typical examples ofaprotic organic solvents include ethylene carbonate (EC), propylenecarbonate, dimethyl carbonate, diethyl carbonate (DEC), γ-butyrolactone,acetonitrile, dimethoxyethane, tetrahydrofuran, and the like, and one ormore of these materials can be used. When a gelled high-molecularmaterial is used as the solvent of the electrolytic solution, safetyagainst liquid leakage is improved. Furthermore, the storage battery canbe thinner and more lightweight. Typical examples of gelledhigh-molecular materials include a silicone gel, an acrylic gel, anacrylonitrile gel, polyethylene oxide, polypropylene oxide, afluorine-based polymer, and the like. Alternatively, the use of one ormore of ionic liquids (room temperature molten salts) which havefeatures of non-flammability and non-volatility as a solvent of theelectrolytic solution can prevent the storage battery from exploding orcatching fire even when the storage battery internally shorts out or theinternal temperature increases owing to overcharging and others.

Instead of the electrolytic solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including amacromolecular material such as a polyethylene oxide (PEO)-basedmacromolecular material may alternatively be used. When the solidelectrolyte is used, a separator and a spacer are not necessary.Furthermore, the battery can be entirely solidified; therefore, there isno possibility of liquid leakage and thus the safety of the battery isdramatically increased.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolytic solutionin charging and discharging a secondary battery, such as nickel,aluminum, and titanium, an alloy of any of the metals, an alloycontaining any of the metals and another metal (e.g., stainless steel),a stack of any of the metals, a stack including any of the metals andany of the alloys (e.g., a stack of stainless steel and aluminum), or astack including any of the metals and another metal (e.g., a stack ofnickel, iron, and nickel) can be used. The positive electrode can 301and the negative electrode can 302 are electrically connected to thepositive electrode 304 and the negative electrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolytic solution. Then, asillustrated in FIG. 36B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303interposed therebetween. In such a manner, the coin-type storage battery300 can be manufactured.

Here, a current flow in charging a battery will be described withreference to FIG. 36C. When a battery using lithium is regarded as aclosed circuit, lithium ions transfer and a current flows in the samedirection. Note that in the battery using lithium, an anode and acathode change places in charge and discharge, and an oxidation reactionand a reduction reaction occur on the corresponding sides; hence, anelectrode with a high redox potential is called a positive electrode andan electrode with a low redox potential is called a negative electrode.For this reason, in this specification, the positive electrode isreferred to as a “positive electrode” and the negative electrode isreferred to as a “negative electrode” in all the cases where charge isperformed, discharge is performed, a reverse pulse current is supplied,and a charging current is supplied. The use of the terms “anode” and“cathode” related to an oxidation reaction and a reduction reactionmight cause confusion because the anode and the cathode change places atthe time of charging and discharging. Thus, the terms “anode” and“cathode” are not used in this specification. If the term “anode” or“cathode” is used, it should be mentioned that the anode or the cathodeis which of the one at the time of charging or the one at the time ofdischarging and corresponds to which of a positive electrode or anegative electrode.

Two terminals in FIG. 36C are connected to a charger, and a storagebattery 400 is charged. As the charge of the storage battery 400proceeds, a potential difference between electrodes increases. Thepositive direction in FIG. 36C is the direction in which a current flowsfrom one terminal outside the storage battery 400 to a positiveelectrode 402, flows from the positive electrode 402 to a negativeelectrode 404 in the storage battery 400, and flows from the negativeelectrode 404 to the other terminal outside the storage battery 400. Inother words, a current flows in the direction of a flow of a chargingcurrent.

[Cylindrical Storage Battery]

Next, an example of a cylindrical storage battery will be described withreference to FIGS. 37A and 37B. As illustrated in FIG. 37A, acylindrical storage battery 600 includes a positive electrode cap(battery cap) 601 on the top surface and a battery can (outer can) 602on the side surface and bottom surface. The positive electrode cap 601and the battery can 602 are insulated from each other by a gasket(insulating gasket) 610.

FIG. 37B is a diagram schematically illustrating a cross section of thecylindrical storage battery. Inside the battery can 602 having a hollowcylindrical shape, a battery element in which a strip-like positiveelectrode 604 and a strip-like negative electrode 606 are wound with astripe-like separator 605 interposed therebetween is provided. Althoughnot illustrated, the battery element is wound around a center pin. Oneend of the battery can 602 is close and the other end thereof is open.For the battery can 602, a metal having corrosion resistance to anelectrolytic solution, such as nickel, aluminum, or titanium, an alloyof such a metal, or an alloy of such a metal and another metal (e.g.,stainless steel) can be used. Alternatively, the battery can 602 ispreferably covered with a corrosive metal such as nickel, aluminum, orthe like in order to prevent corrosion caused by an electrolyticsolution. For example, plating is used. Inside the battery can 602, thebattery element in which the positive electrode, the negative electrode,and the separator are wound is provided between a pair of insulatingplates 608 and 609 which face each other. Inside the battery can 602,the battery element in which the positive electrode, the negativeelectrode, and the separator are wound is interposed between a pair ofinsulating plates 608 and 609 which face each other. Furthermore, anonaqueous electrolytic solution (not illustrated) is injected insidethe battery can 602 provided with the battery element. As the nonaqueouselectrolytic solution, a nonaqueous electrolytic solution which issimilar to that of the above coin-type storage battery can be used.

Although the positive electrode 604 and the negative electrode 606 canbe formed in a manner similar to that of the positive electrode and thenegative electrode of the coin-type storage battery described above, thedifference lies in that, since the positive electrode and the negativeelectrode of the cylindrical storage battery are wound, active materialsare formed on both sides of the current collectors. A positive electrodeterminal (positive electrode current collecting lead) 603 is connectedto the positive electrode 604, and a negative electrode terminal(negative electrode current collecting lead) 607 is connected to thenegative electrode 606. Both the positive electrode terminal 603 and thenegative electrode terminal 607 can be formed using a metal materialsuch as aluminum. The positive electrode terminal 603 and the negativeelectrode terminal 607 are resistance-welded to a safety valve mechanism612 and the bottom of the battery can 602, respectively. The safetyvalve mechanism 612 is electrically connected to the positive electrodecap 601 through a positive temperature coefficient (PTC) element 611.The safety valve mechanism 612 cuts off electrical connection betweenthe positive electrode cap 601 and the positive electrode 604 when theinternal pressure of the battery exceeds a predetermined thresholdvalue. The PTC element 611, which serves as a thermally sensitiveresistor whose resistance increases as temperature rises, limits theamount of current by increasing the resistance, in order to preventabnormal heat generation. Note that barium titanate (BaTiO₃)-basedsemiconductor ceramic can be used for the PTC element.

[Laminated Storage Battery]

Next, an example of a laminated storage battery will be described withreference to FIG. 38A. When a flexible laminated storage battery is usedin an electronic device at least part of which is flexible, the storagebattery can be bent as the electronic device is bent.

A laminated storage battery 500 illustrated in FIG. 38A includes apositive electrode 503 including a positive electrode current collector501 and a positive electrode active material layer 502, a negativeelectrode 506 including a negative electrode current collector 504 and anegative electrode active material layer 505, a separator 507, anelectrolytic solution 508, and an exterior body 509. The separator 507is provided between the positive electrode 503 and the negativeelectrode 506 in the exterior body 509. The exterior body 509 is filledwith the electrolytic solution 508. Any of the positive electrode activematerials described in Embodiment 1 can be used for the positiveelectrode active material layer 502.

In the laminated storage battery 500 illustrated in FIG. 38A, thepositive electrode current collector 501 and the negative electrodecurrent collector 504 also serve as terminals for an electrical contactwith an external portion. For this reason, each of the positiveelectrode current collector 501 and the negative electrode currentcollector 504 is arranged so that part of the positive electrode currentcollector 501 and part of the negative electrode current collector 504are exposed to the outside the exterior body 509. Alternatively, a leadelectrode and the positive electrode current collector 501 or thenegative electrode current collector 504 may be bonded to each other byultrasonic welding, and instead of the positive electrode currentcollector 501 and the negative electrode current collector 504, the leadelectrode may be exposed to the outside the exterior body 509.

As the exterior body 509 in the laminated storage battery 500, forexample, a laminate film having a three-layer structure in which ahighly flexible metal thin film of aluminum, stainless steel, copper,nickel, or the like is provided over a film formed of a material such aspolyethylene, polypropylene, polycarbonate, ionomer, or polyamide, andan insulating synthetic resin film of a polyamide-based resin, apolyester-based resin, or the like is provided as the outer surface ofthe exterior body over the metal thin film can be used. With such athree-layer structure, permeation of the electrolytic solution and a gascan be blocked and an insulating property can be obtained.

FIG. 38B illustrates an example of the cross-sectional structure of thelaminated storage battery 500. Although FIG. 38A illustrates an exampleof including only two current collectors for simplicity, the actualbattery includes more electrode layers.

The example in FIG. 38B includes 16 electrode layers. The laminatedstorage battery 500 has flexibility even though including 16 electrodelayers. In FIG. 38B, 8 negative electrode current collectors 504 and 8positive electrode current collectors 501 are included. Note that FIG.38B illustrates a cross section of the lead portion of the negativeelectrode, and 8 negative electrode current collectors 504 are bonded toeach other by ultrasonic welding. It is needless to say that the numberof electrode layers is not limited to 16, and may be more than 16 orless than 16. In the case of a large number of electrode layers, thestorage battery can have high capacity. In contrast, in the case of asmall number of electrode layers, the storage battery can have smallthickness and high flexibility.

FIGS. 46 and 47 each illustrate an example of the external view of thelaminated storage battery 500. In FIGS. 46 and 47, the positiveelectrode 503, the negative electrode 506, the separator 507, theexterior body 509, a positive electrode lead 510, and a negativeelectrode lead 511 are included.

FIG. 48A illustrates the external views of the positive electrode 503and the negative electrode 506. The positive electrode 503 includes thepositive electrode current collector 501, and the positive electrodeactive material layer 502 is formed on a surface of the positiveelectrode current collector 501. The positive electrode 503 alsoincludes an exposed region of the positive electrode current collector501 (hereinafter, also referred to as a tab region). The negativeelectrode 506 includes the negative electrode current collector 504, andthe negative electrode active material layer 505 is formed on a surfaceof the negative electrode current collector 504. The negative electrode506 also includes an exposed region of the negative electrode currentcollector 504, that is, a tab region. The areas and the shapes of thetab regions included in the positive electrode and the negativeelectrode are not limited to those illustrated in FIG. 48A.

[Method for Forming Laminated Storage Battery]

An example of a method for forming the laminated storage battery whoseexternal view is illustrated in FIG. 46 will be described with referenceto FIGS. 48B and 48C.

First, the negative electrode 506, the separator 507, and the positiveelectrode 503 are stacked. FIG. 48B illustrates a stack including thenegative electrode 506, the separator 507, and the positive electrode503. The battery described here as an example includes 5 negativeelectrodes and 4 positive electrodes. Next, the tab regions of thepositive electrodes 503 are bonded to each other, and the tab region ofthe positive electrode on the outermost surface and the positiveelectrode lead 510 are bonded to each other. The bonding can beperformed by ultrasonic welding, for example. In addition, the tabregions of the negative electrodes 506 are bonded to each other, and thetab region of the negative electrode on the outermost surface and thenegative electrode lead 511 are bonded to each other.

After that, the negative electrode 506, the separator 507, and thepositive electrode 503 are placed over the exterior body 509.

Subsequently, the exterior body 509 is folded along a dashed line asillustrated in FIG. 48C. Then, the outer edge of the exterior body 509is bonded. The bonding can be performed by thermocompression, forexample. At this time, a part (or one side) of the exterior body 509 isleft unbonded (to provide an inlet) so that the electrolytic solution508 can be introduced later.

Next, the electrolytic solution 508 is introduced into the exterior body509 from the inlet of the exterior body 509. The electrolytic solution508 is preferably introduced in a reduced pressure atmosphere or in aninert gas atmosphere. Lastly, the inlet is bonded. In the above manner,the laminated storage battery 500 can be formed.

Note that in this embodiment, the coin-type storage battery, thelaminated storage battery, and the cylindrical storage battery are givenas examples of the storage battery; however, any of storage batterieswith a variety of shapes, such as a sealed storage battery and asquare-type storage battery, can be used. Furthermore, a structure inwhich a plurality of positive electrodes, a plurality of negativeelectrodes, and a plurality of separators are stacked or wound may beemployed.

For each of the positive electrodes of the storage batteries 300, 500,and 600, which are described in this embodiment, the positive electrodeactive layer of one embodiment of the present invention is used. Thus,the discharge capacity of the storage batteries 300, 500, and 600 can beincreased.

[Examples of Electronic Devices]

FIGS. 39A to 39E illustrate examples of electronic devices includingflexible laminated storage batteries. Examples of an electronic deviceincluding a flexible power storage device include television devices(also referred to as televisions or television receivers), monitors ofcomputers or the like, cameras such as digital cameras or digital videocameras, digital photo frames, mobile phones (also referred to as mobilephones or mobile phone devices), portable game machines, portableinformation terminals, audio reproducing devices, large game machinessuch as pachinko machines, and the like.

In addition, a flexible power storage device can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of a car.

FIG. 39A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a power storage device 7407.

The mobile phone 7400 illustrated in FIG. 39B is bent. When the wholemobile phone 7400 is bent by the external force, the power storagedevice 7407 included in the mobile phone 7400 is also bent. FIG. 39Cillustrates the bent power storage device 7407. The power storage device7407 is a laminated storage battery.

FIG. 39D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a power storage device 7104. FIG. 39Eillustrates the bent power storage device 7104.

[Structural Example of Power Storage Device]

Structural examples of power storage devices (storage batteries) will bedescribed with reference to FIGS. 40A and 40B, FIGS. 41A1, 41A2, 41B1,and 41B2, FIGS. 42A and 42B, FIGS. 43A and 43B, and FIG. 44.

FIGS. 40A and 40B are external views of the power storage device. Thepower storage device includes a circuit board 900 and a power storageunit 913. A label 910 is attached to the power storage unit 913.Further, as shown in FIG. 40B, the power storage device includes aterminal 951 and a terminal 952, and includes an antenna 914 and anantenna 915 between the power storage unit 913 and the label 910.

The circuit board 900 includes terminals 911 and a circuit 912. Theterminals 911 are connected to the terminals 951 and 952, the antennas914 and 915, and the circuit 912. Note that a plurality of terminals 911serving as a control signal input terminal, a power supply terminal, andthe like may be provided.

The circuit 912 may be provided on the rear side of the circuit board900. Note that each of the antennas 914 and 915 is not limited to havinga coil shape and may have a linear shape or a plate shape. Further, aplanar antenna, an aperture antenna, a traveling-wave antenna, an EHantenna, a magnetic-field antenna, or a dielectric antenna may be used.Alternatively, the antenna 914 or the antenna 915 may be a flat-plateconductor. The flat-plate conductor can serve as one of conductors forelectric field coupling. That is, the antenna 914 or the antenna 915 canserve as one of two conductors of a capacitor. Thus, power can betransmitted and received not only by an electromagnetic field or amagnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The power storage device includes a layer 916 between the power storageunit 913 and the antennas 914 and 915. The layer 916 has a function ofpreventing the power storage unit 913 from shielding an electromagneticfield. As the layer 916, for example, a magnetic body can be used. Thelayer 916 may serve as a shielding layer.

Note that the structure of the power storage device is not limited tothat shown in FIGS. 40A and 40B.

For example, as shown in FIGS. 41A1 and 41A2, two opposing surfaces ofthe power storage unit 913 in FIGS. 40A and 40B may be provided withrespective antennas. FIG. 41A1 is an external view showing one side ofthe opposing surfaces, and FIG. 41A2 is an external view showing theother side of the opposing surfaces. Note that for portions similar tothose in FIGS. 40A and 40B, description on the power storage deviceshown in FIGS. 40A and 40B can be referred to as appropriate.

As shown in FIG. 41A1, the antenna 914 is provided on one of theopposing surfaces of the power storage unit 913 with the layer 916provided therebetween, and as shown in FIG. 41A2, the antenna 915 isprovided on the other of the opposing surfaces of the power storage unit913 with a layer 917 provided therebetween. The layer 917 has a functionof preventing the power storage unit 913 from shielding anelectromagnetic field. As the layer 917, for example, a magnetic bodycan be used. The layer 917 may serve as a shielding layer.

With the above structure, both of the antennas 914 and 915 can beincreased in size.

Alternatively, as shown in FIGS. 41B1 and 41B2, two opposing surfaces ofthe power storage unit 913 in FIGS. 40A and 40B may be provided withdifferent types of antennas. FIG. 41B1 is an external view showing oneside of the opposing surfaces, and FIG. 41B2 is an external view showingthe other side of the opposing surfaces. Note that for portions similarto those in FIGS. 40A and 40B, description on the power storage deviceshown in FIGS. 40A and 40B can be referred to as appropriate.

As shown in FIG. 41B1, the antennas 914 and 915 are provided on one ofthe opposing surfaces of the power storage unit 913 with the layer 916provided therebetween, and as shown in FIG. 41B2, an antenna 918 isprovided on the other of the opposing surfaces of the power storage unit913 with the layer 917 provided therebetween. The antenna 918 has afunction of performing data communication with an external device, forexample. An antenna with a shape that can be applied to the antennas 914and 915, for example, can be used as the antenna 918. As a system forcommunication using the antenna 918 between the power storage device andanother device, a response method which can be used between the powerstorage device and another device, such as NFC, can be employed.

Alternatively, as shown in FIG. 42A, the power storage unit 913 in FIGS.40A and 40B may be provided with a display device 920. The displaydevice 920 is electrically connected to the terminal 911 via a terminal919. It is possible that the label 910 is not provided in a portionwhere the display device 920 is provided. Note that for portions similarto those in FIGS. 40A and 40B, description on the power storage deviceshown in FIGS. 40A and 40B can be referred to as appropriate.

The display device 920 can display, for example, an image showingwhether or not charging is being carried out, an image showing theamount of stored power, or the like. As the display device 920,electronic paper, a liquid crystal display device, an electroluminescent(EL) display device, or the like can be used. For example, powerconsumption of the display device 920 can be reduced when electronicpaper is used.

Alternatively, as shown in FIG. 42B, the power storage unit 913 in FIGS.40A and 40B may be provided with a sensor 921. The sensor 921 iselectrically connected to the terminal 911 via a terminal 922. Note thatthe sensor 921 may be provided between the power storage unit 913 andthe label 910. Note that for portions similar to those in FIGS. 40A and40B, description on the power storage device shown in FIGS. 40A and 40Bcan be referred to as appropriate.

The sensor 921 has a function of measuring displacement, position,speed, acceleration, angular velocity, rotational frequency, distance,light, liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, electric current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredrays. With the sensor 921, for example, data on an environment (e.g.,temperature) where the power storage device is placed can be detectedand stored in a memory inside the circuit 912.

Further, structural examples of the power storage unit 913 are describedwith reference to FIGS. 43A and 43B and FIG. 44.

The power storage unit 913 shown in FIG. 43A includes a wound body 950provided with the terminals 951 and 952 inside a housing 930. The woundbody 950 is soaked in an electrolytic solution inside the housing 930.The terminal 952 is in contact with the housing 930. An insulator or thelike prevents contact between the terminal 951 and the housing 930. Notethat in FIG. 43A, the housing 930 divided into two pieces is illustratedfor convenience; however, in the actual structure, the wound body 950 iscovered with the housing 930 and the terminals 951 and 952 extend to theoutside of the housing 930. For the housing 930, a metal material (e.g.,aluminum) or a resin material can be used.

Note that as shown in FIG. 43B, the housing 930 in FIG. 43A may beformed using a plurality of materials. For example, in the power storageunit 913 in FIG. 43B, a housing 930 a and a housing 930 b are attachedto each other and the wound body 950 is provided in a region surroundedby the housing 930 a and the housing 930 b.

For the housing 930 a, an insulating material such as an organic resincan be used. In particular, when a material such as an organic resin isused for the side on which an antenna is formed, shielding of anelectric field by the power storage unit 913 can be prevented. Note thatwhen the effect of electric field shielding by the housing 930 a is low,an antenna such as the antennas 914 and 915 may be provided inside thehousing 930. For the housing 930 b, a metal material can be used, forexample.

FIG. 44 shows a structure of the wound body 950. The wound body 950includes a negative electrode 931, a positive electrode 932, and aseparator 933. The wound body 950 is obtained by winding a sheet of astack in which the negative electrode 931 overlaps with the positiveelectrode 932 with the separator 933 provided therebetween. Note that aplurality of layers each including the negative electrode 931, thepositive electrode 932, and the separator 933 may be stacked.

The negative electrode 931 is connected to the terminal 911 in FIGS. 40Aand 40B via one of the terminals 951 and 952. The positive electrode 932is connected to the terminal 911 in FIGS. 40A and 40B via the other ofthe terminals 951 and 952.

[Examples of Electric Devices: Vehicles]

Next, examples where a storage battery is used in a vehicle aredescribed. The use of storage batteries in vehicles can lead tonext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 45A and 45B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8100 illustrated inFIG. 45A is an electric vehicle which runs on the power of the electricmotor. Alternatively, the automobile 8100 is a hybrid electric vehiclecapable of driving using either the electric motor or the engine asappropriate. One embodiment of the present invention achieves ahigh-mileage vehicle. The automobile 8100 includes the power storagedevice. The power storage device is used not only for driving theelectric motor, but also for supplying electric power to alight-emitting device such as a headlight 8101 or a room light (notillustrated).

The power storage device can also supply electric power to a displaydevice included in the automobile 8100, such as a speedometer or atachometer. Furthermore, the power storage device can supply electricpower to a semiconductor device included in the automobile 8100, such asa navigation system.

FIG. 45B illustrates an automobile 8200 including the power storagedevice. The automobile 8200 can be charged when the power storage deviceis supplied with electric power through external charging equipment by aplug-in system, a contactless power supply system, or the like. In FIG.45B, the power storage device included in the automobile 8200 is chargedwith the use of a ground-based charging apparatus 8021 through a cable8022. In charging, a given method such as CHAdeMO (registered trademark)or Combined Charging System may be referred to for a charging method,the standard of a connector, or the like as appropriate. The chargingapparatus 8021 may be a charging station provided in a commerce facilityor a power source in a house. For example, with the use of a plug-intechnique, the power storage device included in the automobile 8200 canbe charged by being supplied with electric power from outside. Thecharging can be performed by converting AC electric power into DCelectric power through a converter such as an AC-DC converter.

Further, although not illustrated, the vehicle may include a powerreceiving device so as to be charged by being supplied with electricpower from an above-ground power transmitting device in a contactlessmanner. In the case of the contactless power supply system, by fittingthe power transmitting device in a road or an exterior wall, chargingcan be performed not only when the automobile stops but also when moves.In addition, the contactless power supply system may be utilized toperform transmission/reception between vehicles. Furthermore, a solarcell may be provided in the exterior of the automobile to charge thepower storage device when the automobile stops or moves. To supplyelectric power in such a contactless manner, an electromagneticinduction method or a magnetic resonance method can be used.

According to one embodiment of the present invention, the power storagedevice can have improved cycle characteristics and reliability.Furthermore, according to one embodiment of the present invention, thepower storage device itself can be made more compact and lightweight asa result of improved characteristics of the power storage device. Thecompact and lightweight power storage device contributes to a reductionin the weight of a vehicle, and thus increases the driving distance.Further, the power storage device included in the vehicle can be used asa power source for supplying electric power to products other than thevehicle. In that case, the use of a commercial power supply can beavoided at peak time of electric power demand.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Example 1

In this example, a lithium-manganese composite oxide was synthesized inaccordance with Embodiment 1, and the composition of the obtainedlithium-manganese composite oxide was measured by inductively coupledplasma-mass spectrometry (ICP-MS) and transmission electronmicroscopy-energy dispersive X-ray spectroscopy (TEM-EDX).

A procedure for synthesizing a lithium-manganese composite oxide will bedescribed in detail below.

The weighed materials, a zirconia ball with a diameter of 3 mm, andacetone were put into a pot made of zirconia, and wet ball milling usinga planetary ball mill was performed at 400 rpm for 2 hours (Step 1).

Then, acetone in slurry subjected to the ball milling was volatilized at50° C. in the air to obtain a mixed material (Step 2).

Next, the mixed material was dried and an alumina crucible was filledwith the dried material, and firing was performed at 1100° C. in the airfor 10 hours to obtain an objective (Step 3).

Subsequently, grinding was performed to separate the sintered particles.The fired material, zirconia balls with diameters of 3 mm and 10 mm, andacetone were put into a pot made of zirconia, and wet ball milling usinga planetary ball mill was performed at 400 rpm for 2 hours (Step 4).

The ground slurry was heated at 50° C. in the air to volatilize acetone(Step 5). Then, vacuum drying was performed (Step 6). Through the abovesteps, a lithium-manganese composite oxide, which is a positiveelectrode active material of an electrode, was synthesized. The obtainedlithium-manganese composite oxide is called Sample A. The measurementresults of the composition of the synthesized lithium-manganesecomposite oxide (i.e., Sample A) by ICP-MS and TEM-EDX are shown in therow (a) in Table 5. Table 5 shows elemental compositions. Values in thecolumn “Mn/Ni” in Table 5 are obtained by dividing the composition of Mnby the composition of Ni, and values in the column “(Mn+Ni)/O” areobtained by dividing the sum of the composition of Mn and thecomposition of Ni by the composition of O. Note that values inparentheses are obtained by normalizing by the amount of raw materialMn, 0.8062.

TABLE 5 (Mn + Analysis method Li Mn Ni O Mn/Ni Ni)/O (a) After ICP-MS59.8 28.2 12.1 — 2.331 — synthesis (1.710) (0.8062) (0.346) TEM-EDX —19.70 8.97 71.35 2.196 0.402 Average value of (0.8062) (0.367) (2.920)measurements at 6 points. (b) After ICP-MS — — — — — — charge andTEM-EDX — 20.04 8.50 71.48 2.358 0.399 discharge Average value of(0.8062) (0.342) (2.876) measurements at 5 points.

The row (a) in Table 5 shows that the atomic ratio of Ni to Mn is highas compared with a feed ratio of raw materials of Li₂CO₃:MnCO₃:NiO,0.84:0.8062:0.318.

Example 2

In this example, Sample A obtained in Example 1 was used as a positiveelectrode active material, and the composition ratio of Mn to Ni and Oin the lithium-manganese composite oxide was analyzed by TEM-EDX aftercharging and discharging.

First, with the use of a positive electrode active material, aconductive additive, a binder, and a disperse medium, a positiveelectrode paste was formed. The positive electrode paste was applied ona positive electrode current collector and dried. Thus, a positiveelectrode including a positive electrode active material layer wasformed.

In this example, the lithium-manganese composite oxide was used as thepositive electrode active material, acetylene black was used as theconductive additive, and polyvinylidene fluoride (PVdF) was used as thebinder. Lithium iron phosphate, acetylene black, and polyvinylidenefluoride were mixed in a ratio of 80:15:5. As the disperse medium forviscosity adjustment, NMP was added to and mixed with the mixture. Thus,the positive electrode paste was formed.

The positive electrode paste formed by the above method was applied tothe positive electrode current collector (20-μm-thick aluminum) anddried at 80° C. for 40 minutes, and then dried at 170° C. in a reducedpressure environment for 10 hours, whereby the positive electrode activematerial layer was formed.

Next, a half cell including the positive electrode was formed and wascharged and discharged. The evaluation was performed using a coin cell.In the coin cell, a lithium metal was used for a negative electrode,polypropylene (PP) was used for a separator, and an electrolyticsolution was formed in such a manner that lithium hexafluorophosphate(LiPF₆) was dissolved at a concentration of 1 mol/L in a solution inwhich ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed ata volume ratio of 1:1 was used. Charging was performed at a constantcurrent and a rate of 0.2 C (it takes five hours for charging) until thevoltage reached a termination voltage of 4.8 V. Discharging wasperformed at a constant current and a rate of 0.2 C (it takes five hoursfor discharging) until the voltage reached a termination voltage of 2 V.The environmental temperature was set at 25° C.

The coin cell was decomposed after the first charge and discharge, andthe composition ratio of Mn to Ni and O in the lithium-manganesecomposite oxide in the positive electrode was analyzed by TEM-EDX. Theresults are shown in the row (b) in Table 5.

The row (b) in Table 5 shows that, after the charge and discharge, aswell as after the synthesis, the atomic ratio of Ni to Mn is high ascompared with the compounding ratio of materials of Li₂CO₃:MnCO₃:NiO,0.84:0.8062:0.318. The comparison between the rows (a) and (b) in Table5 suggests that the charge and discharge might slightly decrease theratio of 0 to Mn.

The compositions before the charge and discharge (i.e., after thesynthesis) and after the charge and discharge can be obtained as shownin the rows (a) and (b) in Table 5. The composition might be changed bycharge and discharge.

Next, a half cell was formed using Sample A obtained in Example 1 as apositive electrode active material, and was subjected to charge anddischarge twice. The discharge characteristics obtained after the secondcharge and discharge are shown in FIG. 34. FIG. 34 indicates thatfavorable discharge capacity can be obtained even after the secondcharge and discharge. Accordingly, a lithium-manganese composite oxidecan have high capacity even with the composition after charge anddischarge, which is shown in the row (b) in Table 5.

Example 3

A laminated storage battery was formed using the lithium-manganesecomposite oxide described in Embodiment 1 as a positive electrode activematerial.

A procedure for synthesizing a lithium-manganese composite oxide will bedescribed in detail below.

The weighed materials, a zirconia ball with a diameter of 3 mm, andacetone were put into a pot made of zirconia, and wet ball milling usinga planetary ball mill was performed at 400 rpm for 2 hours.

Then, acetone in slurry subjected to the ball milling was volatilized at50° C. in the air to obtain a mixed material.

Next, the mixed material was dried and an alumina crucible was filledwith the dried material, and firing was performed at 1100° C. in the airfor 10 hours to obtain an objective.

Subsequently, grinding was performed to separate the sintered particles.The fired material, zirconia balls with diameters of 3 mm and 10 mm, andacetone were put into a pot made of zirconia, and wet ball milling usinga planetary ball mill was performed at 400 rpm for 2 hours.

After the grinding, heating was performed on the mixture subjected towet ball milling at 50° C. in the air to volatilize acetone. Then,heating was performed at 600° C. in the air atmosphere for 3 hours, sothat powdery lithium-manganese composite oxide was obtained. Thepositive electrode 503 was formed using the obtained lithium-manganesecomposite oxide as a positive electrode active material. The thicknessof the formed positive electrode was 85 μm.

Next, with the use of a negative electrode active material, a conductiveauxiliary agent, a binder, and a disperse medium, a negative electrodepaste was formed. The negative electrode paste was applied on thenegative electrode current collector 504 (18-μm-thick copper) and dried.Thus, the negative electrode 506 including the negative electrode activematerial layer 505 was formed. In this example, the negative electrode506 was formed using Ga as a negative electrode active material. Thethickness of the formed negative electrode was 45 μm.

A single-layer laminated storage battery was formed using the obtainedpositive electrode 503 and negative electrode 506. FIG. 38A illustratesthe laminated storage battery 500. The laminated storage battery 500includes the positive electrode 503 including the positive electrodecurrent collector 501 and the positive electrode active material layer502, the negative electrode 506 including the negative electrode currentcollector 504 and the negative electrode active material layer 505, theseparator 507 (25-μm-thick), the electrolytic solution 508, and theexterior body 509. The separator 507 is provided between the positiveelectrode 503 and the negative electrode 506 in the exterior body 509.The exterior body 509 is filled with the electrolytic solution 508. Notethat lead electrodes that are connected to the positive electrodecurrent collector 501 and the negative electrode current collector 504are not illustrated in FIG. 38A.

FIG. 35 shows the discharge characteristics of the formed laminatedstorage battery. The horizontal axis represents the discharge capacitywhich is normalized by the sum of the capacities of the positiveelectrode, the negative electrode, and the separator. Thelithium-manganese composite oxide obtained in one embodiment of thepresent invention was used as a positive electrode active material,whereby a favorable laminated storage battery was able to be formed.

Example 4

In this example, in accordance with the feed ratios of raw materialsshown in Table 6, plural kinds of lithium-manganese composite oxideswere synthesized by the synthesis method described in Embodiment 1. Thesynthesized lithium-manganese composite oxides were subjected to X-raydiffraction measurement and composition analysis.

[1. Synthesis of Lithium-Manganese Composite Oxides]

Lithium-manganese composite oxides represented by Li_(x)Mn_(y)M_(z)O_(w)were synthesized. Table 6 shows raw materials of a comparative sample100 and samples 101 to 109 and the feed ratios of the raw materials. Inthis example, the comparative sample 100 and the samples 101 to 109 wereformed by the combination of the raw materials shown in Table 6. Notethat the synthesis of the sample 105 was repeated four times, and theobtained samples are referred to as a sample 105 a, a sample 105 b, asample 105 c, and a sample 105 d.

TABLE 6 Feed ratio of raw materials Ni/Mn <Comparison Example>Li₂CO₃:MnCO₃ = 0 Sample 100 1:1 Sample 101 Li₂CO₃:MnCO₃ = 0 0.84:1.12Sample 102 Li₂CO₃:MnCO₃:NiO = 0.091 0.84:1.03:0.0937 Sample 103Li₂CO₃:MnCO₃:NiO = 0.176 0.84:0.956:0.168 Sample 104 Li₂CO₃:MnCO₃:NiO =0.276 0.84:0.8812:0.243 Sample 105 Li₂CO₃:MnCO₃:NiO = 0.3940.84:0.8062:0.318 Sample 106 Li₂CO₃:MnCO₃:NiO = 0.537 0.84:0.7312:0.393Sample 107 Li₂CO₃:MnCO₃:NiO = 0.72 0.84:0.65:0.468 Sample 108Li₂CO₃:MnCO₃:NiO = 0.935 0.84:0.581:0.543 Sample 109 Li₂CO₃:MnCO₃:NiO =1.221 0.84:0.506:0.618

First, Li₂CO₃, MnCO₃, and NiO were weighed as starting materials inaccordance with Table 6. Table 6 shows molar ratios. Note that thesample 101 and the comparative sample 100 were formed without containingNiO as a raw material. The comparative sample 100 has the mixture ratioof raw materials for the purpose of obtaining Li₂MnO₃ with a layeredrock-salt structure. Note that in the case where Li₂MnO₃ with a layeredrock-salt structure is formed, the ratio of lithium to manganese is 2:1.In the sample 101, the ratio of lithium to manganese was 1.68:1.12. Inother words, the ratio of lithium to manganese was decreased. In each ofthe samples 102 to 109, part of manganese in the sample 101 was replacedwith the element M (here, Ni).

Next, acetone was added to the powder of these materials, and then, theywere mixed in a ball mill to prepare mixed powder.

After that, heating was performed to volatilize acetone, so that a mixedmaterial was obtained.

Then, the mixed materials were put into a crucible and were fired at1000° C. for 10 hours in the air at a flow rate of 10 L/min., so that anovel material was synthesized.

Subsequently, grinding was performed to separate the sintered particles.For the grinding, acetone was added and then mixing was performed in aball mill.

After the grinding, heating was performed to volatilize acetone, andthen, vacuum drying was performed.

Next, firing was performed at 600° C. for 3 hours in the air at a flowrate of 10 L/min. Through the above steps, powdery novel material wasobtained.

The comparative sample 100 and the samples 101 to 109 were formed in thesame manner except for the feed ratios of raw materials.

[2. X-Ray Diffraction]

Next, the lithium-manganese composite oxides synthesized using the rawmaterials shown in Table 6 were subjected to X-ray diffraction (XRD)measurement. FIG. 15 shows the measurement results. FIGS. 21 to 23 areenlarged views of FIG. 15.

FIG. 15 and FIGS. 21 to 23 show that two peaks at around 21° and around22° are found in the comparative sample 100, the sample 101, and thesample 102, whereas those peaks tend to be weak in the samples 103 to109. The two peaks are unique to Li₂MnO₃ with a layered rock-saltstructure that belongs to the space group C12/m1. As suggested by theresults of the Rietveld analysis described later, the higher the ratioof Ni to Mn (Ni/Mn) in a layered rock-salt structure is, the greater thesum of the occupancy of Mn and the occupancy of Ni is at three sites:the 2b site, the 2c site, and the 4h site. In Li₂MnO₃ with a layeredrock-salt structure that belongs to the space group C12/m1, the twopeaks might be weakened because the periodicity of the crystal isdisturbed when a Ni atom or a Mn atom is substituted at any of the 2bsite, the 2c site, and the 4h site, which are Li sites.

In the samples 101 and 102, a peak at around 36° can be observed inaddition to a peak at around 37°. This might be due to LiMn₂O₄ with aspinel structure that belongs to the space group Fd-3m. The peak isweakened in other samples probably because the proportion of the spinelstructure is small as suggested by the results of the Rietveld analysisdescribed later.

FIGS. 24A and 24B, FIGS. 25A and 25B, and FIG. 26 show enlarged views ofthe measurement results of X-ray diffraction (2θ is in the range from25° to 50°) performed on the samples 102, 103, 105 c, 106, and 108.)

Table 7 and Table 8 show the values of 2θ and the intensities of someobvious peaks obtained by X-ray diffraction. Table 7 shows the resultsof Peaks 1 to 5, and Table 8 shows the results of Peaks 6 to 9. Notethat I1 shown in Table 7 and Table 8 is the peak intensity, and I2 isthe ratio of the intensity of each peak to the intensity of Peak 1. Thepeaks without data do not always mean that no peak was observed; thesome data are not shown because the intensity is low (200 counts orlower). Peak 1 has the maximum value where 2θ is in the range from 18.6°to 18.8°, and is probably assigned to a (001) plane. Peak 2 has themaximum value where 2θ is in the range from 20.65° to 20.90°, and isprobably assigned to a (020) plane. Peak 3 has the maximum value where2θ is in the range from 21.5° to 21.85°, and is probably assigned to a(110) plane. Peak 4 is probably due to the spinel structure as describedabove. Peak 5 has the maximum value where 2θ is in the range from 36° to37.5°, and is probably assigned to a (130) plane. Peak 6 has the maximumvalue where 2θ is in the range from 37.8° to 39.3°. Peak 7 has themaximum value where 2θ is in the range from 43.7° to 44.7°. Peak 8 hasthe maximum value where 2θ is in the range from 43.8° to 45.3°. Peak 9has the maximum value where 2θ is in the range from 48° to 49.5°.

When the ratio of Ni to Mn (Ni/Mn, which is the feed ratio of rawmaterials) is greater than or equal to 0.276 (in the case of the samples104 to 109), the ratio of the intensity of Peak 6 to the intensity ofPeak 1 (I2) becomes greater than or equal to 0.05. To obtain highcapacity, I2 of Peak 6 is preferably greater than or equal to 0.04.

Furthermore, I2 of Peak 9 also becomes greater than or equal to 0.052 inthe samples 104 to 109. To obtain high capacity, I2 of Peak 9 ispreferably greater than or equal to 0.04. In other words, the maximumvalue where 2θ is in the range from 18.6° to 18.8° to the maximum valuewhere 2θ is in the range from 48° to 49.5° is preferably greater than orequal to 0.04.

TABLE 7 Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Sample 2θ [°] 18.74 20.8521.80 36.34 37.06 101 I1 6509 616 513 469 974 I2 — 0.095 0.079 0.0720.150 Sample 2θ [°] 18.68 20.81 21.73 36.42 36.98 102 I1 6890 629 340378 1511 I2 — 0.091 0.049 0.055 0.219 Sample 2θ [°] 18.66 20.81 21.73 —36.94 103 I1 6350 493 244 — 1387 I2 — 0.078 0.038 — 0.218 Sample 2θ [°]18.66 20.71 21.6 — 36.81 104 I1 5931 412.8 211.6 — 1664 I2 — 0.070 0.036— 0.281 Sample 2θ [°] 18.69 20.7 — — 36.82 105c I1 6832 408 — — 1624 I2— 0.060 — — 0.238 Sample 2θ [°] 18.73 20.77 — — 36.83 106 I1 4776 224 —— 1138 I2 — 0.046965 — — 0.238 Sample 2θ [°] 18.71 — — — 36.77 108 I12040 — — — 499 I2 — — — — 0.245 Sample 2θ [°] 18.72 — — — 36.68 109 I15480 — — — 1236 I2 — — — — 0.226

TABLE 8 Peak 6 Peak 7 Peak 8 Peak 9 Sample 101 2θ [°] — 44.13 44.83 — I1— 437 2166 — I2 — 0.067 0.333 — Sample 102 2θ [°] 38.62 — 44.70 — I1 265— 2459 — I2 0.038 — 0.357 — Sample 103 2θ [°] 38.61 — 44.65 48.75 I1 222— 2576 300 I2 0.035 — 0.406 0.047 Sample 104 2θ [°] 38.42 — 44.53 48.66I1 387 — 3379 383 I2 0.065 — 0.570 0.065 Sample 105c 2θ [°] 38.43 —44.53 48.65 I1 342 — 3229 358 I2 0.050 — 0.473 0.052 Sample 106 2θ [°]38.46 — 44.55 48.65 I1 316 — 2430 321 I2 0.066 — 0.509 0.067 Sample 1082θ [°] 38.34 — 44.43 48.59 I1 222 — 1465 208 I2 0.109 — 0.718 0.102Sample 109 2θ [°] 38.30 — 44.37 48.55 I1 499 — 3408 395 I2 0.091 — 0.6220.072

[3. Rietveld Analysis 1]

Next, the crystal structure data acquired by the Rietveld analysis isdescribed below. Fitting was performed under the conditions where theinitial first phase was Li₂MnO₃ with a layered rock-salt structure thatbelongs to the space group C12/m1 and the initial second phase wasLiMn₂O₄ with a spinel structure that belongs to the space group Fd-3m.Since the analysis conditions were the same as those described inEmbodiment 1, the detailed description is omitted here.

The weight proportion of a layered rock-salt structure to a spinelstructure was calculated by the Rietveld analysis. Note that the initialstates for the calculation were basically the same as those in Table 1and Table 2. The lattice constants of the sample 105 c were calculatedusing the values in Table 1 and Table 2 as the initial values, and thelattice constants of other samples were calculated using the latticeconstants of the sample 105 c as the initial values. The latticeconstants of the sample 105 c were as follows: a=0.4959 nm, b=0.8583 nm,and c=0.5033 nm. Each site was analyzed on the assumption that the 2bsites in a layered rock-salt structure are occupied by Li and Mn. Notethat as described later, Mn and Ni are hard to distinguish in X-raydiffraction. The calculation was started with an occupancy of Li at the2b sites of 70% and an occupancy of Mn at the 2b sites of 30% as theinitial values. It was assumed that other sites in the layered rock-saltstructure were occupied by the atoms shown in Table 1 at 100%, and thesites in the spinel structure were occupied by the atoms shown in Table2 at 100%.

Table 9 shows weight proportions in the first phase and the second phaseobtained by the Rietveld analysis.

TABLE 9 Weight proportion of Sample Ni/Mn spinel structure [%] Sample101 0.000 16.00  Sample 102 0.091 9.52 Sample 103 0.176 6.39 Sample 1040.276 0.03 Sample 105a 0.394 Not calculated Sample 105b 0.394 1.07Sample 105c 0.394 0.84 Sample 105d 0.394 Not calculated Sample 106 0.5370.63 Sample 107 0.720 Not calculated Sample 108 0.935 0.04 Sample 1091.221 0.02

As shown in Table 9, in the samples having Ni/Mn (the feed ratio of rawmaterials) of greater than or equal to 0.276, the weight proportions ofthe spinel structures were lower than or equal to approximately 1.1%.

[4. Rietveld Analysis 2]

Next, for more detailed examination of the occupancies of atoms at eachsite in a layered rock-salt crystal structure, calculation is performedfor the occupancies of Li, Mn, and Ni at four sites: the 2b site, the 2csite, the 4h site, and the 4g site. Here, to calculate the occupanciesof the atoms and the lattice constants, the Rietveld analysis isperformed on the assumption of a single layer of a layered rock-saltstructure.

The initial states for the calculation were basically the same as thosein Table 1 and Table 2. The lattice constants of the sample 105 c werecalculated using the values in Table 1 and Table 2 as the initialvalues, and the lattice constants of other samples were calculated usingthe lattice constants of the sample 105 c as the initial values. Thelattice constants of the samples 105 are shown in Table 10 below.

The calculation results of the samples 105 a to 105 d obtained by theRietveld analysis are shown in Table 10, for example. Because of thesmall difference of the X-ray scattering power between Ni and Mn, Ni andMn are hard to distinguish. For this reason, the sum of the occupancy ofNi and the occupancy of Mn is discussed here.

TABLE 10 Sample Sample Sample Sample 105a 105b 105c 105d a(Å) 4.9594.959 4.956 4.948 b(Å) 8.601 8.595 8.591 8.599 c(Å) 5.031 5.031 5.0285.025 β(°) 109.1 109.1 109.1 109.0 Sum of filling factor 4g site 92.1%89.4% 89.6% 85.7% of Mn and filling 2b site 63.0% 62.6% 63.7% 62.8%factor of Ni [%] 2c site 1.9% 6.1% 0.4% 0.0% 4h site 6.4% 11.0% 4.3%2.0% Rexp 3.33 3.70 3.14 3.13 Rwp 4.35 4.45 4.45 5.03 Rp 3.29 3.42 3.203.43 GOF 1.30 1.20 1.42 1.61

Here, Rwp is obtained by dividing the residual sum of squares by the sumtotal of the observed intensities, and Rp is a difference between theobserved intensities and the theoretical diffraction intensities.Furthermore, Rexp is the expected value of Rwp, which is thestatistically estimated minimum Rwp. In addition, GOF, which stands for“good of fitness”, is obtained by dividing Rwp by Rexp and is preferablyclose to 1.

Focusing on the sum of the occupancy of Ni and the occupancy of Mn inLi₂MnO₃, the 2b site (the initial coordinates: (0, 0.5, 0)), the 2c site(the initial coordinates: (0, 0, 0.5)), and the 4h site (the initialcoordinates: (0, 0.6560, 0)), which are Li sites, are examined. Table 10shows that 63.0% of the 2b sites in the sample 105 a are occupied by Niand Mn. In addition, Table 10 shows that 1.9% of the 2c sites and 6.4%of the 4h sites were are occupied by Ni and Mn.

FIG. 16 shows the sum of A(Ni)_(2b) and A(Mn)_(2b),[A(Ni)_(2b)+A(Mn)_(2b)], and FIG. 17 shows the sum of A(Ni)_(2c−4h) andA(Mn)_(2c+4h), [A(Ni)_(2c−4h)+A(Mn)_(2c+4h)], of a layered rock-saltstructure, which belongs to the space group C12/m1 and which is thefirst phase, of the samples 101 to 109. The horizontal axis representsNi/Mn (the feed ratio of raw materials) of each sample. Note that thesample 107 was not analyzed.

First, in a region where Ni/Mn (the feed ratio of raw materials) is lessthan 0.2, the 2b sites are occupied by Ni and Mn. In a region whereNi/Mn is greater than 0.2, the occupancies at the 2c site and the 4hsites are increased. Thus, it can be considered that the 2b sites, the2c sites, and the 4h sites are occupied by Ni and Mn when the capacityis increased, that is, when Ni/Mn (the feed ratio of raw materials) is0.2758 (equivalent to the conditions of the sample 104) or greater.Here, A(Ni)_(2b)+A(Mn)_(2b) is preferably greater than or equal to 40%,further preferably greater than or equal to 40% and less than or equalto 85%, still further preferably greater than or equal to 40% and lessthan or equal to 75%. In addition, A(Ni)_(2c+4h)+A(Mn)_(2c+4h) ispreferably greater than or equal to 0.2%, further preferably greaterthan or equal to 0.5%.

Consideration is given to the reason why the capacity is increased inthe case where Ni/Mn (the feed ratio of raw materials) is 0.2758(equivalent to the conditions of the sample 104) or greater. In theregion where Ni/Mn is less than 0.2, the sum of the occupancies of Niand Mn is increased not only at the 2b sites but also at the 2c site andthe 4h site. Since at least one of the 2b site, the 2c site, and the 4hsite is occupied by Ni or Mn, crystal distortion or a change in electronstate occurs, for example; thus, Li might be easily diffused.

FIG. 18, FIG. 19, and FIG. 20 respectively show lattice constants a, b,and c of the samples 101 to 109. The horizontal axis represents Ni/Mn(the feed ratio of raw materials) of each sample. A straight lineobtained by linear approximation is shown in each of FIGS. 18 to 20. Theapproximate line was obtained by using three points where Ni/Mn are 0,0.935, and 1.221. As an example where the lattice constant is linearlychanged, the case where Vegard's law is obeyed can be given. In general,Vegard's law is obeyed in solid solutions, and the lattice constant islinearly changed depending on the concentration of the solution.

FIGS. 18 and 19 show that when the capacity starts to increase, that is,when Ni/Mn (the feed ratio of raw materials) is around 0.2758(equivalent to the conditions of the sample 104), the lattice constantsa and b are likely to be outside the approximate line. It is assumedthat Ni is not dissolved in a region outside the approximate line, forexample. The lattice constant a is preferably larger than or equal to0.494 nm, further preferably larger than or equal to 0.494 nm andsmaller than or equal to 0.4975 nm, still further preferably larger thanor equal to 0.494 nm and smaller than or equal to 0.4965 nm. The latticeconstant b is preferably larger than or equal to 0.856 nm, furtherpreferably larger than or equal to 0.856 nm and smaller than or equal to0.864 nm, still further preferably larger than or equal to 0.856 nm andsmaller than or equal to 0.862 nm.

FIG. 20 shows that the lattice constant c is significantly increasedwhen Ni/Mn (the feed ratio of raw materials) is larger than or equal to0.0915. The lattice constant c is preferably larger than or equal to0.5021 nm, further preferably larger than or equal to 0.5021 nm andsmaller than or equal to 0.5038 nm, still further preferably larger thanor equal to 0.5021 nm and smaller than or equal to 0.5035 nm.

Example 5

In this example, a half cell was formed using the lithium-manganesecomposite oxide synthesized in Example 4, and the dischargecharacteristics were examined.

[1. Fabrication of Cell]

A half cell was fabricated using an electrode containing thelithium-manganese composite oxide synthesized in Example 4, and chargeand discharge characteristics were measured. Note that in the half cell,a lithium-manganese composite oxide was used for a positive electrodeand lithium metal was used for a negative electrode.

Here, the operation of the half cell is described. FIG. 11 shows thecase of charging the half cell, and FIG. 12 shows the case ofdischarging the half cell.

FIG. 11 illustrates the connection between a lithium-ion secondarybattery 141 that is charged and a charger 142. In the case of chargingthe lithium-ion secondary battery, a reaction of Formula (9) occurs at apositive electrode.

Li_(x)Mn_(y)M_(z)O_(w)→Li_(x-a)Mn_(y)M_(z)O_(w) +aLi⁺ +ae ⁻  (9)

In addition, a reaction of Formula (10) occurs in the negativeelectrode.

aLi⁺ +ae ⁻ →aLi  (10)

In Formula (9) and Formula (10), 0<a<x is satisfied.

FIG. 12 illustrates the connection between a lithium-ion secondarybattery 141 that is discharged and a load 143. In the case ofdischarging the lithium-ion secondary battery, a reaction of Formula(11) occurs at a positive electrode.

Li_(x-a)Mn_(y)M_(z)O_(w) +aLi⁺ +ae ⁻→Li_(x)Mn_(y)M_(z)O_(w)  (11)

In addition, a reaction of Formula (12) occurs in the negativeelectrode.

aLi→aLi⁺ +ae ⁻  (12)

In Formula (11) and Formula (12), 0<a<x is satisfied.

First, a lithium-manganese composite oxide synthesized using the rawmaterials shown in Table 1, a PVdF resin, and AB as a conductiveadditive were dissolved in N-methyl-2-pyrrolidon (NMP), a polar solvent,and were mixed to form slurry. The compounding ratio of thelithium-manganese composite oxide to AB and PVdF was adjusted to be80:15:5 in weight ratio. Then, the slurry was applied on a currentcollector and dried. Note that a surface of the current collector wascovered with an undercoat in advance. Here, the “undercoat” refers to afilm formed over a current collector before applying a positiveelectrode paste onto the current collector for the purpose of reducingthe interface resistance between an active material layer and thecurrent collector or increasing the adhesion between the active materiallayer and the current collector. Note that the undercoat is notnecessarily formed in a film shape, and may be formed in an islandshape. For the undercoat, a carbon material can be used, for example.Examples of the carbon material are graphite, carbon black such asacetylene black or ketjen black, and carbon nanotubes.

Forming the undercoat over the current collector can reduce theresistance at the interface between the current collector and the activematerial layer formed later, and/or can increase adhesion between theactive material layer and the current collector. The undercoat ispreferably not dissolved by a reducing solution in the process ofreducing graphene oxide. Note that if there is no problem with theadhesion between the current collector and the active material layer,the electrode strength, and the interface resistance between the currentcollector and the electrode, it is not necessary to apply the undercoatto the current collector.

A lithium metal was used for a negative electrode and a space between apositive electrode and the negative electrode was filled with anelectrolytic solution, so that the half cell was fabricated. Note thatthe electrolytic solution was formed by dissolving LiPF₆ as a salt in amixed solution containing ethylene carbonate (EC) and diethyl carbonate(DEC), which were aprotic organic solvents, at a volume ratio of 1:1. Asthe separator, polypropylene (PP) was used.

[2. Discharge Characteristics Examination]

FIG. 13 shows the discharge capacities of half cells including theformed samples 100 to 109. The vertical axis represents voltage (V), andthe horizontal axis represents discharge capacity (mAh/g). Charging wasperformed at a constant current with a current density of 30 mA/g untilthe voltage reached a termination voltage of 4.8 V. Discharging wasperformed at a constant current with a current density of 30 mA/g untilthe voltage reached a termination voltage of 2.0 V. The temperatureduring the charge and discharge measurement was 25° C.

FIG. 14 shows a graph in which the raw material feed ratios of Ni to Mnare plotted on the horizontal axis and the discharge capacities areplotted on the vertical axis, in accordance with FIG. 13. Note that thedischarge capacity of the comparative sample 100 is not plotted here. Inthe samples whose Ni/Mn (the feed ratio of raw materials) is 0.2758(equivalent to the conditions of the sample 104) or greater, thecapacity is high.

The ratio of the Li composition to the sum of the Ni composition and theMn composition in Li_(x)Mn_(y)M_(z)O_(w) is represented by x/(y+z), andis preferably less than 2, further preferably less than 1.6. Inaddition, the ratio of the Ni composition to the Mn composition isrepresented by z/y, and the ratio of the Li composition to the Mncomposition is represented by x/y.

Example 6

In this example, in accordance with the feed ratios of raw materialsshown in Table 11, plural kinds of lithium-manganese composite oxideswere synthesized by the synthesis method described in Embodiment 1.

[1. Synthesis of Lithium-Manganese Composite Oxides]

Lithium-manganese composite oxides represented by Li_(x)Mn_(y)M_(z)O_(w)were synthesized. Table 11 shows raw materials of samples 121 to 131 andthe molar ratios of the raw materials. Table 11 shows values Li/(Mn+Ni)obtained by dividing the molar quantity of lithium (raw material) by thesum of the molar quantity of manganese (raw material) and the molarquantity of nickel (raw material). Table 11 also shows values Ni/Mnobtained by dividing the molar quantity of nickel (raw material) by themolar quantity of manganese (raw material).

TABLE 11 Li₂CO₃:MnCO₃:NiO (Molar ratio) Li/(Mn + Ni) Ni/Mn Sample 1210.64:0.857:0.428 1.00 0.50 Sample 122 0.7838:1.125:0.12 1.26 0.11 Sample123 0.7838:1.0313:0.2138 1.26 0.21 Sample 124 0.77:0.8545:0.327 1.300.38 Sample 125 0.8:0.8062:0.318 1.42 0.39 Sample 126 0.92:0.8:0.36 1.590.45 Sample 127 0.92:1:0.16 1.59 0.16 Sample 128 0.924:0.8062:0.318 1.640.39 Sample 129 0.915:0.8:0.26 1.73 0.33 Sample 130 0.915:0.58:0.48 1.730.83 Sample 131 0.915:0.89:0.17 1.73 0.19

First, Li₂CO₃, MnCO₃, and NiO were weighed as starting materials inaccordance with Table 11.

Next, acetone was added to the powder of these materials, and then, theywere mixed in a ball mill to prepare mixed powder.

After that, heating was performed to volatilize acetone, so that a mixedmaterial was obtained.

Then, the mixed materials were put into a crucible and were fired at1000° C. for 10 hours in the air at a flow rate of 10 L/min., so that anovel material was synthesized.

Subsequently, grinding was performed to separate the sintered particles.For the grinding, acetone was added and then mixing was performed in aball mill.

After the grinding, heating was performed to volatilize acetone, andthen, vacuum drying was performed.

Through the above steps, powdery novel material was obtained.

Next, half cells were formed using the samples 121 to 131, and thedischarge characteristics were examined.

[2. Fabrication of Cell]

Half cells were fabricated using an electrode including the samples 121to 131, and charge and discharge characteristics were measured. Notethat in the half cell, a lithium-manganese composite oxide was used fora positive electrode and lithium metal was used for a negativeelectrode.

A method for forming the electrodes is described. First, a PVdF resin,AB for a conductive additive, and each of lithium-manganese compositeoxides synthesized using the raw materials shown in Table 11 weredissolved in N-methyl-2-pyrrolidon (NMP), a polar solvent, and weremixed to form slurry. Table 12 shows the compounding ratios of thelithium-manganese composite oxides (“active material” in Table 12) to ABand PVdF in weight ratios. Then, the slurry was applied on a currentcollector, and then is dried at 80° C. to form the electrodes. Afterthat, the electrodes were heated at respective temperatures shown inTable 12.

TABLE 12 Composition [weight %] Heating temperature Activematerial:AB:PVDF of electrode Sample 121 80:15:5 170° C. Sample 12290:5:5 250° C. Sample 123 90:5:5 250° C. Sample 124 80:15:5 170° C.Sample 125 90:5:5 250° C. Sample 126 80:15:5 170° C. Sample 127 90:5:5250° C. Sample 128 90:5:5 250° C. Sample 129 80:15:5 170° C. Sample 13080:15:5 170° C. Sample 131 80:15:5 170° C.

A surface of the current collector was covered with an undercoat inadvance. Here, the “undercoat” refers to a film formed over a currentcollector before applying a positive electrode paste onto the currentcollector for the purpose of reducing the interface resistance betweenan active material layer and the current collector or increasing theadhesion between the active material layer and the current collector.Note that the undercoat is not necessarily formed in a film shape, andmay be formed in an island shape. For the undercoat, a carbon materialcan be used, for example. Examples of the carbon material are graphite,carbon black such as acetylene black or ketjen black, and carbonnanotubes.

Forming the undercoat over the current collector can reduce theresistance at the interface between the current collector and the activematerial layer formed later, and/or can increase adhesion between theactive material layer and the current collector. The undercoat ispreferably not dissolved by a reducing solution in the process ofreducing graphene oxide. Note that if there is no problem with theadhesion between the current collector and the active material layer,the electrode strength, and the interface resistance between the currentcollector and the electrode, it is not necessary to apply the undercoatto the current collector.

A lithium metal was used for a negative electrode and a space between apositive electrode and the negative electrode was filled with anelectrolytic solution, so that the half cell was fabricated. Note thatthe electrolytic solution was formed by dissolving LiPF₆ as a salt in amixed solution containing ethylene carbonate (EC) and diethyl carbonate(DEC), which were aprotic organic solvents, at a volume ratio of 1:1. Asthe separator, polypropylene (PP) was used.

[3. Discharge Characteristics Examination]

FIGS. 49A to 49C are graphs in which the discharge capacities of halfcells including the samples 121 to 131 are plotted on the vertical axisand the raw material feed ratios of Ni to Mn are plotted on thehorizontal axis. FIG. 49A shows the discharge capacities that can beobtained in the case of using the samples with which Li/(Mn+Ni) are1.00, 1.26, and 1.30 in Table 11. FIG. 49B shows the dischargecapacities that can be obtained in the case of using the samples withwhich Li/(Mn+Ni) are 1.42 and 1.59 in Table 11. Note that the results ofthe case where Li/(Mn+Ni) is 1.49 in FIG. 49B were plotted using thedata shown in FIG. 13 in Example 5. FIG. 49C shows the dischargecapacities that can be obtained in the case of using the samples withwhich Li/(Mn+Ni) are 1.64 and 1.73 in Table 11.

From FIGS. 49A and 49B, the case where Li/(Ni+Mn) is greater than orequal to 1.46 is preferred to the case where Li/(Ni+Mn) is less than orequal to 1.30 to achieve high capacity. In addition, the results in FIG.49C indicate that high discharge capacity can be obtained whenLi/(Ni+Mn) is 1.64 or 1.73.

Next, examination is performed focusing on Ni/Mn. The comparison betweenFIGS. 49B and 49C indicates that the range of Ni/Mn with which highcapacitance can be obtained is larger in the case where Li/(Ni+Mn) is1.49 than in the case where Li/(Ni+Mn) is 1.73.

Accordingly, in a lithium-manganese composite oxide represented byLi_(D)Mn_(y)Ni_(z)O_(w), D/(y+z) is preferably greater than or equal to1.35 and less than 2, further preferably greater than or equal to 1.4and less than 1.8, still further preferably greater than or equal to 1.4and less than 1.6. In addition, 0.2<z/y<1.2 is preferably satisfied.Note that in storage batteries, lithium is released from alithium-manganese composite oxide by charge or the like, for example.Here, D denotes, for example, the amount of lithium contained in alithium-manganese composite oxide before the lithium is released bycharge or the like, or the amount of lithium contained in alithium-manganese composite oxide after lithium is released by chargeand inserted by discharge or the like.

EXPLANATION OF REFERENCE

-   -   100: comparative sample, 101: sample, 102: sample, 103: sample,        104: sample, 105: sample, 105 a: sample, 105 b: sample, 105 c:        sample, 105 d: sample, 106: sample, 107: sample, 108: sample,        109: sample, 121: sample, 122: sample, 123: sample, 124: sample,        125: sample, 126: sample, 127: sample, 128: sample, 129: sample,        130: sample, 131: sample, 141: lithium-ion secondary battery,        142: charger, 143: load, 150: comparative sample, 151: sample,        152: sample, 153: sample, 154: sample, 155: sample, 156: sample,        157: sample, 158: sample, 159: sample, 160: sample, 161: sample,        162: sample, 163: sample, 164: sample, 165: sample, 166: sample,        170: part, 201: spinel crystallite, 202: layered rock-salt        crystallite, 203: Li₂MnO₃ particle, 204: Spi-LiMn₂O₄ particle,        205: sintered material, 300: storage battery, 301: positive        electrode can, 302: negative electrode can, 303: gasket, 304:        positive electrode, 305: positive electrode current collector,        306: positive electrode active material layer, 307: negative        electrode, 308: negative electrode current collector, 309:        negative electrode active material layer, 310: separator, 400:        storage battery, 402: positive electrode, 404: negative        electrode, 500: storage battery, 501: positive electrode current        collector, 502: positive electrode active material layer, 503:        positive electrode, 504: negative electrode current collector,        505: negative electrode active material layer, 506: negative        electrode, 507: separator, 508: electrolytic solution, 509:        exterior body, 600: storage battery, 601: positive electrode        cap, 602: battery can, 603: positive electrode terminal, 604:        positive electrode, 605: separator, 606: negative electrode,        607: negative electrode terminal, 608: insulating plate, 609:        insulating plate, 610: gasket, 611: PTC element, 612: safety        valve mechanism, 900: circuit board, 910: label, 911: terminal,        912: circuit, 913: power storage unit, 914: antenna, 915:        antenna, 916: layer, 917: layer, 918: antenna, 919: terminal,        920: display device, 921: sensor, 922: terminal, 930: housing,        930 a: housing, 930 b: housing, 931: negative electrode, 932:        positive electrode, 933: separator, 951: terminal, 952:        terminal, 7100: portable display device, 7101: housing, 7102:        display portion, 7103: operation button, 7104: power storage        device, 7400: mobile phone, 7401: housing, 7402: display        portion, 7403: operation button, 7404: external connection port,        7405: speaker, 7406: microphone, 7407: power storage device,        8021: charging device, 8022: cable, 8024: power storage device,        8100: automobile, 8101: headlight, and 8200: automobile.

This application is based on Japanese Patent Application serial no.2013-147170 filed with Japan Patent Office on Jul. 15, 2013 and JapanesePatent Application serial no. 2013-198871 filed with Japan Patent Officeon Sep. 25, 2013, the entire contents of which are hereby incorporatedby reference.

1. (canceled)
 2. A lithium-manganese composite oxide represented byLi_(x)Mn_(y)Ni_(z)O_(w), wherein x is greater than or equal to zero,wherein y, z, and w are each greater than zero, wherein x, y, z, and wsatisfy 1.4≦x/(y+z)≦1.64, 0.26≦(y+z)/w<0.5 and 0.2<z/y<1.2, wherein thelayered rock-salt crystal structure belongs to a space group C12/m1,wherein in the layered rock-salt crystal structure, the sum ofA(Mn)_(2b) and A(Ni)_(2b) is greater than or equal to 40%, and whereinA(Mn)_(2b) is an occupancy of Mn at the 2b site, and wherein A(Ni)_(2b)is an occupancy of Ni at the 2b site, wherein in the layered rock-saltcrystal structure, the sum of A(Mn)_(2c+4h) and A(Ni)_(2c+4h) is greaterthan or equal to 0.2%, wherein A(Mn)_(2c+4h) is an occupancy of Mn at a2c site and an occupancy of Mn at a 4h site, represented by Formula (1),wherein A(Ni)_(2c+4h) is an occupancy of Ni at the 2c site and anoccupancy of Ni at the 4h site, A(Ni)_(2c+4h), represented by Formula(2), and wherein A(Mn)_(2c) is the occupancy of Mn at the 2c site,A(Ni)_(2c) is the occupancy of Ni at the 2c site, A(Mn)_(4h) is theoccupancy of Mn at the 4h site, and A(Ni)_(4h) is the occupancy of Ni atthe 4h site.A(Mn)_(2c+4h) =[A(Mn)_(2c)×1+A(Mn)_(4h)×2]÷(1+2)  Formula (1)A(Ni)_(2c+4h) =[A(Ni)_(2c)×1+A(Ni)_(4h)×2]÷(1+2)  Formula (2)
 3. Thelithium-manganese composite oxide according to claim 2, wherein thelithium-manganese composite oxide comprises a layered rock-salt crystalstructure and a spinel crystal structure.
 4. The lithium-manganesecomposite oxide according to claim 2, wherein in the layered rock-saltcrystal structure, an a-axis lattice constant is larger than or equal to0.494 nm and a b-axis lattice constant is larger than or equal to 0.856nm.
 5. The lithium-manganese composite oxide according to claim 2,wherein a c-axis lattice constant of the layered rock-salt crystalstructure is larger than or equal to 0.5021 nm.
 6. A lithium-ionsecondary battery comprising the lithium-manganese composite oxideaccording to claim 2, as a positive electrode active material.
 7. Anelectric device comprising the lithium-ion secondary battery accordingto claim
 6. 8. A lithium-manganese composite oxide represented byLi_(x)Mn_(y)Ni_(z)O_(w), wherein x is greater than or equal to zero,wherein y, z, and w are each greater than zero, wherein x, y, z, and wsatisfy 1.4≦x/(y+z)≦1.64, 0.26≦(y+z)/w<0.5 and 0.2<z/y<1.2, wherein thelayered rock-salt crystal structure belongs to a space group C12/m1, andwherein in the layered rock-salt crystal structure, an a-axis latticeconstant is larger than or equal to 0.494 nm and a b-axis latticeconstant is larger than or equal to 0.856 nm.
 9. The lithium-manganesecomposite oxide according to claim 8, wherein a c-axis lattice constantof the layered rock-salt crystal structure is larger than or equal to0.5021 nm.
 10. The lithium-manganese composite oxide according to claim8, wherein the lithium-manganese composite oxide comprises a layeredrock-salt crystal structure and a spinel crystal structure.
 11. Alithium-ion secondary battery comprising the lithium-manganese compositeoxide according to claim 8, as a positive electrode active material. 12.An electric device comprising the lithium-ion secondary batteryaccording to claim 11.